tests: add goleak leak detection and advisory race-detector CI (CON-179)

[prakhargarg105]

Part of CON-179 (test hardening). This PR adds two complementary checks: goroutine-leak detection inside the test suites of selected connector packages, and a race-detector CI job that starts out advisory and gains teeth package by package.

1. Goroutine-leak detection (goleak)

Adds go.uber.org/goleak and wires it into a package-level TestMain for four packages: internal/impl/mysql, internal/impl/postgresql, internal/impl/protobuf, and internal/impl/sql. After the last test in the package finishes, goleak fails the run if any goroutine started during the tests is still alive.

How it runs: TestMain is Go's standard per-package entry point, so this needs no CI wiring. It fires on every go test of these packages, which means every PR (via the existing test.yml unit suite) and every nightly integration run. A leak is a hard test failure.

Ignore policy: each TestMain carries a small set of narrow, individually commented ignores for goroutines we cannot stop from test code. Examples: prototransform's SchemaWatcher poll loop (its owner exposes no stop hook and the processor's Close is a no-op), and the license service's hourly expiry-metric loop started by InjectTestService. Broad ignores are deliberately avoided; each entry names the library and the reason.

Real leak fixed along the way: TestIntegrationPostgresSnapshotAckBarrier blocked a consumer goroutine on a context that benthos never cancels (consumer funcs receive context.Background()), which leaked the goroutine and the unacked stream behind it past the end of the test. The test now uses its own cancellable context for the simulated crash and explicitly stops the crashed stream with StopWithin. The protobuf mock BSR server also gets a proper Shutdown on cleanup.

All four packages were verified leak-clean on repeated local runs before the check was enabled, so this cannot fail a PR on a pre-existing condition.

2. Advisory race-detector CI job

New workflow .github/workflows/race_test.yml, plus a promotion list at .github/race-blocking-packages.txt.

How it runs: on every PR the job diffs against the merge-base and collects the internal/impl/<pkg> directories the PR touched (same auto-scoping as integration_test.yml, which keeps the 2-10x race-detector runtime cost bounded). If no connector package changed, the job skips entirely. Otherwise it runs go test -count=1 -race -shuffle=on -timeout 10m per touched package and writes a pass/fail table to the job summary. A workflow_dispatch trigger allows manual runs against any package by name.

Advisory-first design: a race failure produces a warning annotation, and the check stays green, unless the failing package is listed in race-blocking-packages.txt. That file starts empty, so on day one nothing can go red from this job. A package is promoted to blocking only after it holds a green -race baseline (repeated clean scoped runs plus a green advisory run in CI). The invariant this preserves: a blocking gate must never be able to fail a PR on a pre-existing race the PR did not introduce.

Supporting changes

• CONTRIBUTING.md: contributors are asked to run task test:unit-race on packages they touch, with a pointer to the blocking-list semantics.
• .claude/agents/tester.md: recipe for wiring goleak into a new package, including the ignore policy and the requirement to verify against a clean baseline before merging.
• go.mod: adds go.uber.org/goleak v1.3.0 (test-only dependency).

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[prakhargarg105]

Superseded by #4721 — remade from a branch directly on this repo instead of a fork.