Complete final test infrastructure cleanup

[yyjeqhc]

Summary

Final follow-up to the test cleanup merged in #127 and #130. This branch is based directly on current main at 5fa2aafb (Consolidate redundant runtime test coverage (#130)) and contains four focused cleanup/governance commits:

• bc27d324 Consolidate Runner execution lifecycle tests
• 6fb9a11b Consolidate Runner LSP synchronization tests
• b09a6bfb Complete test governance cleanup
• 8eb21911 Codify test governance rules

The remaining work closes the test-infrastructure debt that was intentionally left after the earlier cleanup rounds:

• consolidate Runner detached-job, JobManager, polling/streaming transport lifecycle fixtures and bounded waits
• share the Runner fake-LSP binary and absolute-deadline readiness helpers across supervisor/navigation tests
• consolidate Windows UIA test synchronization without changing Computer runtime behavior
• replace scattered server-side process-environment mutation with one panic-safe, restoring TestEnvGuard
• refresh the cheap CI contract lane to current canonical OpenAPI/MCP compatibility tests instead of stale/redundant ownership
• document canonical test placement, test-domain ownership, and wait_* vs probe semantics for future development

No production runtime behavior change is intended. Changes in production-source files are test-only/test-support adjustments; the non-test changes are CI/test-governance documentation.

Review performed

I independently reviewed this branch before opening the PR rather than relying only on green tests.

• Compared test-function ownership between origin/main and this HEAD across all changed Rust files. Only one test was removed outright: utf_encoding_public_conversions.
• Verified that its UTF-8/UTF-16/UTF-32 + emoji round-trip semantics are already owned more strongly by the dedicated lsp/position.rs unit suite, including Chinese/emoji offsets and invalid mid-codepoint positions.
• The WebSocket disconnect test is not lost; it is renamed/strengthened from notifier-removal wording to the current contract: client goes offline while its record is retained.
• Checked new Runner lifecycle helpers for bounded absolute deadlines / bounded join behavior instead of fixed-yield readiness assumptions.
• Checked the shared server TestEnvGuard against the canonical process-global env lock and audited changed tests for obvious nested non-reentrant lock use; no new deadlock path was found.
• Verified every test name retained by the CI contract lane exists and has a canonical current owner. The consolidated MCP parity test now covers legacy + stateless-2026 and both compact-schema modes.

No correctness blocker was found and no additional review-fix commit was necessary.

Validation

On exact HEAD 8eb21911dfdad404b8bef9ca8a1a3dbcade1843d:

• cargo fmt --all -- --check — passed
• git diff --check origin/main...HEAD — passed
• git diff --check — passed
• bash scripts/test_inventory.sh --self-test — 2 passed
• bash scripts/test_inventory.sh — passed
• CI contract-lane canonical tests — 6/6 passed
• cargo test -p webcodex — 2770 passed / 0 failed / 0 ignored
• cargo test -p webcodex-runner — 743 passed / 0 failed / 3 ignored
• cargo test -p webcodex-admin — 17 passed / 0 failed / 0 ignored

The branch is clean, exactly four commits ahead of current main, and already synchronized to origin/chore/test-cleanup.

Native macOS/Windows CI lanes remain enabled in the workflow and will provide the platform-specific PR validation, including the Windows-only UIA test consolidation.