Add helper for job metadata updates

[peter941221]

1. Adds a small public SetMetadata(ctx, key, value) helper for writing metadata updates onto the existing work-context merge path.
2. Keeps the surface intentionally narrow: no batch API, no delete API, no hook signature changes, and no completion semantics changes.
3. Rejects river:-prefixed keys so user code can't collide with River-reserved metadata keys.
4. Adds focused coverage for the helper contract itself plus worker and HookWorkEnd integration paths.

Closes #1218.

Verification:

1. go test ./... -run "TestSetMetadata|Test_Client_Common/(WithWorkerSettingMetadata|WithWorkEndHookSettingMetadata)" -count=1
2. make test

[brandur]

Cool! Seems useful. I like the touch that we reject use of the river:* namespace.

[peter941221]

Thanks. I dropped the unrelated CLI and worker.go changes, scoped the test-only job args into the test case, and normalized the error message casing.

[peter941221]

One note on the remaining red check: the retriggered run moved the failure from build_and_test (1.26, 15) to build_and_test (1.26, 14), and both failures happened during Docker pull for postgres:15 in container setup rather than during test execution, so this looks like CI flakiness rather than a code regression.\n\nLatest run: https://github.com/riverqueue/river/actions/runs/26804256615