test(leak): grow stack dump dynamically and cover wedged pipeline paths

[hammadmajid]

Description

Fixes the silent stack truncation vulnerability in the goroutine leak detector and adds coverage for wedged pipeline and aggregator paths that were previously untested.

Closes #19

Type of Change

[✓] Bug fix (non-breaking change fixing an issue)
[ ] New feature (non-breaking change adding functionality)
[ ] Performance improvement
[ ] Refactoring or code cleanup
[ ] Documentation update
[ ] CI/CD or build workflow change

Key Changes

Growable Stack Dump: runtime.Stack silently truncates when the dump exceeds the buffer capacity, returning len(buf) with no error. A test binary with many live goroutines can easily exceed the previous fixed 128 KiB buffer, causing verifyNoGoroutineLeaks to pass vacuously.

• Added fullStackDump() which dynamically doubles the buffer starting from 64 KiB until n < len(buf), guaranteeing the complete stack dump is captured without truncation.
• Factored out grgGoroutinesInDump(raw string) []string and filtered internal testing / leak-checking stack frames (fullStackDump, grgGoroutinesInDump, awaitNoGRGGoroutines, requireNoLingeringGoroutines).
• Added requireNoLingeringGoroutines(t, what) to assert no grg goroutines remain mid-test while holding an uncancelled context (proving the component self-unwound rather than relying on deferred cancellation).

Expanded Test Coverage:

• TestLeakDetector_GrowsBufferUntilDumpFits: Proves that when 64 goroutines are parked at depth 64, the dump exceeds 128 KiB and the fixed buffer underreports the count (17 of 64), while fullStackDump() captures all 64.
• TestGoroutineLeak_SearchPipeline_AbandonWithoutDraining: Exercises abandon without cancel by reading 1 result from ExecuteContext, abandoning without draining until workers park on resultsCh <- res, then cancelling and asserting all goroutines unwind.
• TestGoroutineLeak_SearchPipeline_FatalErrorAbandonsRun: Asserts that a fatal error halts dispatch and stops workers without searching all remaining blobs or leaking goroutines.
• TestGoroutineLeak_SearchPipeline_PanickingTask: Asserts panic containment in a task surfaces as an error from Execute without leaking goroutines.
• TestGoroutineLeak_AggregateChannel_RealPipelineFatalError: Tests AggregateChannel driven by a real pipeline where a poisoned blob explodes while the results buffer is full and workers are blocked on sends. Asserts AggregateChannel draining resultsCh successfully unwinds the pipeline.
• TestGoroutineLeak_AggregateChannel_RealPipelineCancelled: Tests AggregateChannel driven by a real pipeline with mid-stream context cancellation, asserting early termination, correct error propagation, and clean pipeline rundown.

Verification & Testing

[✓] Ran go test -v -count=1 ./...
[✓] Ran go test -race -shuffle=on -count=1 ./...
[✓] Ran go vet ./...
[✓] Added or updated unit/integration tests
[✓] Tested manually against sample Git repository histories

  go build ./...                                      clean
  go vet ./...                                        clean
  gofmt -l                                            empty
  go test -race -shuffle=on -count=1 ./test/...       ok
  go test -race -shuffle=on -count=1 ./...            ok

Checklist

[✓] gofmt clean
[✓] Every new test mutation-checked: reverting fixes makes them fail
[✓] Passes the full race suite with -shuffle=on