fix: honor cancellation when the result relay is blocked

[root-Manas]

When an SDK caller stops reading Service.Execute results and cancels its context, a relay blocked on the full output channel cannot exit. Its unconditional send also prevents the waiter from closing the result channel. Agents already use cancellation-aware sends, but the relay still needs the same handling.

Use the existing sources.SendResult helper for the relay. Regression tests exercise cancellation with both an unbuffered output and the default 32-result buffer, and verify that normal delivery preserves order, timestamps, and channel closure. Both cancellation cases fail on the unmodified dev implementation and pass with this change.

Related to #724; this addresses cancellation in the service relay after the producer-side changes.

Validation on Windows/amd64 with Go 1.27.0:

• go test . -run TestExecute -count=20
• go test ./...
• go vet ./...
• go build -o ../artifacts/uncover.exe ./cmd/uncover

Local race testing could not run because this environment has CGO disabled and no C compiler installed.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The reviewed changes address cancellation handling and include comprehensive regression coverage.

Review effort: Lite
Findings: None

What changed in this PR

Updates Service.Execute to honor context cancellation when result delivery is blocked.

Changes:

• Uses sources.SendResult for cancellation-aware relay delivery.
• Adds regression tests for buffered and unbuffered cancellation, ordering, timestamps, and closure.

File	Description
uncover.go	Makes relay output cancellation-aware.
uncover_test.go	Adds cancellation and delivery behavior tests.

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