Reconcile dbconnect with the VPEX CLI spec: single sync command, E_* taxonomy, camelCase JSON

Aligns `databricks dbconnect` with the reconciled cli-spec:
- Collapse `init`+`sync` into a single `dbconnect sync` that auto-detects
  greenfield (no pyproject.toml) vs. merge; command path lives in one constant.
- Add `--constraints-only` (Python + constraints, no databricks-connect pin;
  still builds .venv, omits dbconnectVersion, skips the DB Connect assertion).
- Rewrite the `--output json` contract to the camelCase schema: schemaVersion,
  command, ok, mode, dryRun, target, resolved, greenfield, plan, phases[] (all
  six phases with pending), warnings[], error{code,failurePhase,diskMutated}.
- Rename error codes to the E_* set; report failurePhase at the phase that
  detects the failure so it always matches the errored phase in phases[].
- Detect non-uv managers (conda/pip) in preflight and exit cleanly with
  E_MANAGER_UNSUPPORTED; a plain PEP 621 pyproject.toml resolves to uv.
- Classify a 404 for a resolved env key as E_ENV_UNSUPPORTED (latest-LTS hint,
  no cache fallback) vs. transport failure as E_FETCH; add a writable preflight.
- Default the constraint repo to rugpanov/databricks-environments.

Fixes two bugs the real `uv sync`/validate path exposed (both masked by the
fake package manager and --check in tests):
- uvManager.Validate no longer requires databricks-connect to be importable
  (constraints-only left it uninstalled), so validate stops failing after it
  has already provisioned the venv.
- Greenfield render now emits project.version, which uv requires for a
  [project] table; without it every real greenfield `uv sync` failed.

Co-authored-by: Isaac

Author: rugpanov

NEXT_CHANGELOG.md

@@ -5,7 +5,7 @@
 ### Notable Changes
 
 ### CLI
-* Add `databricks dbconnect init` and `databricks dbconnect sync` to provision a local Python environment (Python version, `databricks-connect` pin, and dependency constraints) matched to the selected Databricks compute target.
+* Add `databricks dbconnect sync` to provision a local Python environment (Python version, `databricks-connect` pin, and dependency constraints) matched to the selected Databricks compute target. A project with no `pyproject.toml` is initialized from scratch; an existing one is merged in place. Supports `--cluster`/`--serverless`/`--job` target selection, `--constraints-only` (skip `databricks-connect`), `--check` (dry run), and `--output json` for the VS Code extension.
 
 ### Bundles
 * `bundle run` now prints the modern job run URL (`/jobs/<id>/runs/<id>`) so that non-admin users permitted to view the run are taken to the run instead of the workspace homepage.

acceptance/dbconnect/cluster-unsupported/output.txt

@@ -0,0 +1,53 @@
+
+>>> [CLI] dbconnect sync --serverless v4 --constraints-only --check --output json
+{
+  "schemaVersion": 1,
+  "command": "dbconnect sync",
+  "ok": true,
+  "mode": "constraints-only",
+  "dryRun": true,
+  "target": {
+    "source": "serverless",
+    "serverlessVersion": "v4",
+    "envKey": "serverless/serverless-v4"
+  },
+  "resolved": {
+    "pythonVersion": "3.12",
+    "artifactSource": "network"
+  },
+  "greenfield": true,
+  "plan": {
+    "wouldWrite": "[TEST_TMP_DIR]/pyproject.toml",
+    "wouldInstallPython": "3.12",
+    "diff": "--- pyproject.toml\n+++ pyproject.toml\n@@ -1 +1,15 @@\n+[project]\n+name = \"001\"\n+version = \"0.0.0\"\n+requires-python = \"\u003e=3.12\"\n+\n+[dependency-groups]\n+dev = []\n+\n+# managed by databricks dbconnect — do not edit\n+[tool.uv]\n+constraint-dependencies = [\n+    \"pyarrow\u003c19\",\n+    \"pandas\u003c3\",\n+]\n+# end managed by databricks dbconnect\n"
+  },
+  "phases": [
+    {
+      "phase": "preflight",
+      "status": "ok"
+    },
+    {
+      "phase": "resolve",
+      "status": "ok"
+    },
+    {
+      "phase": "fetch",
+      "status": "ok"
+    },
+    {
+      "phase": "merge",
+      "status": "ok"
+    },
+    {
+      "phase": "provision",
+      "status": "ok"
+    },
+    {
+      "phase": "validate",
+      "status": "ok"
+    }
+  ],
+  "warnings": [],
+  "error": null,
+  "durationMs": 0
+}

acceptance/dbconnect/cluster-unsupported/script

@@ -0,0 +1 @@
+trace $CLI dbconnect sync --serverless v4 --constraints-only --check --output json

…onnect/cluster-unsupported/out.test.toml …dbconnect/constraints-only/out.test.tomlacceptance/dbconnect/cluster-unsupported/out.test.toml renamed to acceptance/dbconnect/constraints-only/out.test.toml acceptance/dbconnect/cluster-unsupported/out.test.toml renamed to acceptance/dbconnect/constraints-only/out.test.toml

@@ -0,0 +1,21 @@
+EnvMatrix.DATABRICKS_BUNDLE_ENGINE = []
+
+[Env]
+DATABRICKS_DBCONNECT_CONSTRAINT_SOURCE = "$DATABRICKS_HOST"
+
+[[Server]]
+Pattern = "GET /serverless/serverless-v4/pyproject.toml"
+Response.Body = '''
+[project]
+requires-python = ">=3.12"
+
+[dependency-groups]
+dev = ["databricks-connect~=17.2.0"]
+
+[tool.uv]
+constraint-dependencies = ["pyarrow<19", "pandas<3"]
+'''
+
+[[Repls]]
+Old = 'uv uv \S+(?: \([^)]+\))?'
+New = 'uv [UV_VERSION]'

acceptance/dbconnect/constraints-only/output.txt

@@ -0,0 +1,8 @@
+preflight  ok  uv [UV_VERSION]
+resolve    ok  source=cluster envKey=dbr/15.4.x-scala2.12
+fetch      error  no published environment for "dbr/15.4.x-scala2.12". If this is a new runtime, try the latest LTS target (e.g. --serverless v4 or a supported --cluster DBR): GET [DATABRICKS_URL]/dbr/15.4.x-scala2.12/pyproject.toml: environment key not found
+merge      pending
+provision  pending
+validate   pending
+For more detail, re-run with --debug, or --output json to share a structured report.
+Error: no published environment for "dbr/15.4.x-scala2.12". If this is a new runtime, try the latest LTS target (e.g. --serverless v4 or a supported --cluster DBR): GET [DATABRICKS_URL]/dbr/15.4.x-scala2.12/pyproject.toml: environment key not found

acceptance/dbconnect/constraints-only/script

@@ -0,0 +1 @@
+musterr $CLI dbconnect sync --cluster test-cluster-id --check