[PECOBLR-1928] Add AI coding agent detection to User-Agent header (#326)

## Summary
- Adds `internal/agent` package that detects 7 AI coding agents (Claude
Code, Cursor, Gemini CLI, Cline, Codex, OpenCode, Antigravity) by
checking well-known environment variables they set in spawned shell
processes
- Integrates detection into `InitThriftClient` to append
`agent/<product>` to the User-Agent header
- Uses exactly-one detection rule: if zero or multiple agent env vars
are set, no agent is attributed (avoids ambiguity)

## Approach
Mirrors the implementation in
[databricks/cli#4287](databricks/cli#4287) and
aligns with the latest agent list in
[`libs/agent/agent.go`](https://github.com/databricks/cli/blob/main/libs/agent/agent.go#L35).

| Agent | Product String | Environment Variable |
|-------|---------------|---------------------|
| Google Antigravity | `antigravity` | `ANTIGRAVITY_AGENT` |
| Claude Code | `claude-code` | `CLAUDECODE` |
| Cline | `cline` | `CLINE_ACTIVE` |
| OpenAI Codex | `codex` | `CODEX_CI` |
| Cursor | `cursor` | `CURSOR_AGENT` |
| Gemini CLI | `gemini-cli` | `GEMINI_CLI` |
| OpenCode | `opencode` | `OPENCODE` |

Adding a new agent requires only a new constant and a new entry in
`knownAgents`.

## Changes
- **New**: `internal/agent/agent.go` — environment-variable-based agent
detection with injectable env lookup for testability
- **New**: `internal/agent/agent_test.go` — 11 test cases covering all
agents, no agent, multiple agents, empty values, and real `os.Getenv`
- **Modified**: `internal/client/client.go` — calls `agent.Detect()`
when building User-Agent in `InitThriftClient`

## Test plan
- [x] `internal/agent` — 11 tests pass
- [x] `internal/client` — all existing tests continue to pass
- [x] Manual: verified User-Agent contains `agent/claude-code` when run
from Claude Code via `GODEBUG=http2debug=2`
  ```
http2: Transport encoding header "user-agent" =
"godatabrickssqlconnector/1.10.0 agent/claude-code"
  ```
- [x] Executed `SELECT 1` successfully against dogfood warehouse with
the new header

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: Vikrant Puppala <vikrant.puppala@databricks.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: vikrantpuppala

internal/agent/agent.go

@@ -0,0 +1,66 @@
+// Package agent detects whether the Go SQL driver is being invoked by an AI
+// coding agent by checking for well-known environment variables that agents set
+// in their spawned shell processes.
+//
+// Detection only succeeds when exactly one agent environment variable is
+// present, to avoid ambiguous attribution when multiple agent environments
+// overlap.
+//
+// Adding a new agent requires only a new constant and a new entry in
+// knownAgents.
+//
+// References for each environment variable:
+//   - ANTIGRAVITY_AGENT: Closed source. Google Antigravity sets this variable.
+//   - CLAUDECODE: https://github.com/anthropics/claude-code (sets CLAUDECODE=1)
+//   - CLINE_ACTIVE: https://github.com/cline/cline (shipped in v3.24.0)
+//   - CODEX_CI: https://github.com/openai/codex (part of UNIFIED_EXEC_ENV array in codex-rs)
+//   - CURSOR_AGENT: Closed source. Referenced in a gist by johnlindquist.
+//   - GEMINI_CLI: https://google-gemini.github.io/gemini-cli/docs/tools/shell.html (sets GEMINI_CLI=1)
+//   - OPENCODE: https://github.com/opencode-ai/opencode (sets OPENCODE=1)
+package agent
+
+import "os"
+
+const (
+	Antigravity = "antigravity"
+	ClaudeCode  = "claude-code"
+	Cline       = "cline"
+	Codex       = "codex"
+	Cursor      = "cursor"
+	GeminiCLI   = "gemini-cli"
+	OpenCode    = "opencode"
+)
+
+var knownAgents = []struct {
+	envVar  string
+	product string
+}{
+	{"ANTIGRAVITY_AGENT", Antigravity},
+	{"CLAUDECODE", ClaudeCode},
+	{"CLINE_ACTIVE", Cline},
+	{"CODEX_CI", Codex},
+	{"CURSOR_AGENT", Cursor},
+	{"GEMINI_CLI", GeminiCLI},
+	{"OPENCODE", OpenCode},
+}
+
+// Detect returns the product string of the AI coding agent driving the current
+// process, or an empty string if no agent (or multiple agents) are detected.
+func Detect() string {
+	return detect(os.Getenv)
+}
+
+// detect is the internal implementation that accepts an env lookup function
+// for testability.
+func detect(getenv func(string) string) string {
+	var detected []string
+	for _, a := range knownAgents {
+		if getenv(a.envVar) != "" {
+			detected = append(detected, a.product)
+		}
+	}
+	if len(detected) == 1 {
+		return detected[0]
+	}
+	return ""
+}

internal/agent/agent_test.go

@@ -0,0 +1,71 @@
+package agent
+
+import (
+	"testing"
+)
+
+func envWith(vars map[string]string) func(string) string {
+	return func(key string) string {
+		return vars[key]
+	}
+}
+
+func TestDetectsSingleAgent(t *testing.T) {
+	cases := []struct {
+		envVar  string
+		product string
+	}{
+		{"ANTIGRAVITY_AGENT", Antigravity},
+		{"CLAUDECODE", ClaudeCode},
+		{"CLINE_ACTIVE", Cline},
+		{"CODEX_CI", Codex},
+		{"CURSOR_AGENT", Cursor},
+		{"GEMINI_CLI", GeminiCLI},
+		{"OPENCODE", OpenCode},
+	}
+	for _, tc := range cases {
+		t.Run(tc.product, func(t *testing.T) {
+			got := detect(envWith(map[string]string{tc.envVar: "1"}))
+			if got != tc.product {
+				t.Errorf("detect() = %q, want %q", got, tc.product)
+			}
+		})
+	}
+}
+
+func TestReturnsEmptyWhenNoAgent(t *testing.T) {
+	got := detect(envWith(map[string]string{}))
+	if got != "" {
+		t.Errorf("detect() = %q, want empty", got)
+	}
+}
+
+func TestReturnsEmptyWhenMultipleAgents(t *testing.T) {
+	got := detect(envWith(map[string]string{
+		"CLAUDECODE":   "1",
+		"CURSOR_AGENT": "1",
+	}))
+	if got != "" {
+		t.Errorf("detect() = %q, want empty", got)
+	}
+}
+
+func TestIgnoresEmptyValues(t *testing.T) {
+	got := detect(envWith(map[string]string{"CLAUDECODE": ""}))
+	if got != "" {
+		t.Errorf("detect() = %q, want empty", got)
+	}
+}
+
+func TestDetectUsesOsGetenv(t *testing.T) {
+	// Clear all known agent env vars, then set one
+	for _, a := range knownAgents {
+		t.Setenv(a.envVar, "")
+	}
+	t.Setenv("CLAUDECODE", "1")
+
+	got := Detect()
+	if got != ClaudeCode {
+		t.Errorf("Detect() = %q, want %q", got, ClaudeCode)
+	}
+}

internal/client/client.go

@@ -20,6 +20,7 @@ import (
 	"time"
 
 	dbsqlerr "github.com/databricks/databricks-sql-go/errors"
+	"github.com/databricks/databricks-sql-go/internal/agent"
 	dbsqlerrint "github.com/databricks/databricks-sql-go/internal/errors"
 
 	"github.com/apache/thrift/lib/go/thrift"
@@ -295,6 +296,9 @@ func InitThriftClient(cfg *config.Config, httpclient *http.Client) (*ThriftServi
 		if cfg.UserAgentEntry != "" {
 			userAgent = fmt.Sprintf("%s/%s (%s)", cfg.DriverName, cfg.DriverVersion, cfg.UserAgentEntry)
 		}
+		if agentProduct := agent.Detect(); agentProduct != "" {
+			userAgent = fmt.Sprintf("%s agent/%s", userAgent, agentProduct)
+		}
 		thriftHttpClient.SetHeader("User-Agent", userAgent)
 
 	default: