fix(go): preserve coverage output from go test -cover on passing runs

[tylern91]

Summary

• Detect coverage: lines in filter_go_test_json() and route them to a dedicated coverage field on PackageResult, not package_fail_output
• Append per-package coverage lines below the passing summary when present
• Preserves the compact format for runs without -cover

Root Cause

Coverage lines arrive as package-level output JSON events (no Test field). The handler previously pushed all package-level output to package_fail_output, which is only emitted when package_failed == true. On passing runs, coverage data was silently discarded.

Output Format

Go test: 348 passed in 9 packages
  foo: coverage: 80.1% of statements
  bar: coverage: 92.3% of statements

Test plan

• Added test_filter_go_test_cover_output_preserved() verifying coverage lines appear in passing-run output
• cargo fmt --all && cargo clippy --all-targets && cargo test --all — 1871 passed, 0 failures
• Runs without -cover unchanged (coverage vec empty, indented block skipped)

Closes #1765

[CLAassistant]

All committers have signed the CLA.

[tylern91]

@aeppling friendly bump when you have a chance to take a look — thanks!