test(runtime-e2e): add x-client-id/ parity with the other 4 SDKs

[saurabhjain1592]

Summary

Closes the post-merge coherence gap caught by master-tracker review of workstream B of Epic #2230: the Rust SDK was missing the runtime-e2e/x-client-id/ runner shipped by the other 4 SDKs (Go #168, Python #195, TS #227, Java #177).

This PR adds the runner. No SDK code change — src/client.rs is unchanged from v0.3.0. Pure test infrastructure.

Bumps to v0.3.1 (patch).

Files

• runtime-e2e/x-client-id/README.md — what the test verifies + how to run + parity context
• runtime-e2e/x-client-id/test.sh — bash entry point: boots community docker-compose stack, runs the helper, asserts. Same shape as runtime-e2e/anthropic_interceptor/test.sh.
• runtime-e2e/x-client-id/helper/ — small Rust cargo project (binary, publish = false) implementing an in-process forwarding-proxy: binds tokio TcpListener on 127.0.0.1:0, captures the SDK's outbound headers off the wire, forwards the request to AXONFLOW_AGENT_URL via reqwest, asserts the 4 v9 invariants.
• CHANGELOG.md — v0.3.1 entry
• Cargo.toml — version = "0.3.1"

R1 — mechanical

• cargo build --release clean
• cargo clippy --all-targets -- -D warnings clean (SDK)
• cargo clippy --all-targets -- -D warnings --manifest-path runtime-e2e/x-client-id/helper/Cargo.toml clean
• cargo fmt --check clean (SDK + helper)
• cargo test --quiet — 23 passed, 0 failed (existing v0.3.0 wiremock unit tests still pass — additive change)

R2 — functional (real-wire helper run)

Disclosure: Docker Desktop on the session host returned HTTP 500 from its API socket regardless of API version, so test.sh's full docker-compose path could not run end-to-end in this session. CI will exercise that path. To prove the helper logic itself is correct end-to-end, I ran the helper against a Python stand-in HTTP upstream on 127.0.0.1:18081 with full request-capture.

Helper stdout (PASS):

PASS: 4/4 header assertions
  X-Client-ID:       demo-client
  X-Axonflow-Client: sdk-rust/0.3.1
  Authorization:     Basic <redacted base64>
  X-Tenant-ID:       <absent (✓)>

Wire-level headers as received by the upstream (these are the exact bytes the agent would see):

content-type:      application/json
accept:            */*
user-agent:        axonflow-sdk-rust/0.3.1
x-axonflow-client: sdk-rust/0.3.1
authorization:     Basic ZGVtby1jbGllbnQ6ZGVtby1zZWNyZXQ=
x-client-id:       demo-client
host:              127.0.0.1:18081
content-length:    102

Decoding Basic ZGVtby1jbGllbnQ6ZGVtby1zZWNyZXQ= → demo-client:demo-secret ✓ matches AXONFLOW_TENANT_ID:AXONFLOW_TENANT_SECRET.
x-tenant-id is absent ✓ — agent's apiAuthMiddleware still accepts it as an alias for back-compat through v9, but SDK-side we standardize on x-client-id.

• SDK head SHA at branch base: 7db1d76 (post-feat(identity): emit X-Client-ID + X-Axonflow-Client headers (v9) #40 merge)
• Agent ref: docker-unavailable this session — will be captured by CI run

R3 — hostile self-review (per feedback_mutation_test_to_prove_assertion_not_tautological.md)

Mutation test:

Step	Action	Result
1	Backed up src/client.rs → /tmp/client_rs_baseline	OK
2	Replaced headers.insert(\"X-Client-ID\", val); with // MUTATION-TEST: removed X-Client-ID	OK
3	Reran helper (SDK rebuilt from mutated source)	FAIL — exit code 1, stderr: FAIL: X-Client-ID: want \"demo-client\", got None
4	Restored src/client.rs from baseline	OK
5	Reran helper	PASS — exit code 0, all 4 assertions passed

✅ Mutation confirmed: the runtime-e2e assertion fails iff the SDK does not emit X-Client-ID. Assertion is not tautological.

Hostile review pass also checked:

• clippy::doc_overindented_list_items on a multi-line doc-comment list in the helper — fixed in source before commit
• No SDK behaviour change — git diff origin/main -- src/ is empty
• Cross-SDK parity confirmed: Go has main.go, Python has test.py, TS has test.mjs, Java has SdkXClientIdTest.java, Rust now has test.sh + helper/ (matching Rust's existing anthropic_interceptor style)
• Cargo.lock of helper not committed (helper-local .gitignore) — same justification as the comment in that file

Full R3 doc kept at /tmp/v9-rust-e2e-20260519T221516Z/r3.md (not committed per feedback_evidence_bundles_never_in_git.md).

Coordination

• Independent of Phase 5 / Workstream C / Workstream D.
• Per feedback_sub_sessions_admin_merge_when_ci_green_r3_done.md, admin-merging on green CI + R3 done; no master-tracker gating.