Go-SDK: exclude oversized-payload framing test from -race builds

[jason810496]

Why

go test -race ./pkg/execution/ currently fatals before any test in the package can run.

TestWriteFrameRejectsOversizedPayload constructs a 4 GiB+ slice with unsafe.Slice(&backing, MaxFrameSize+1) to exercise writeFrame's size guard cheaply, and the checkptr instrumentation that -race enables treats the resulting "slice straddles allocations" as a fatal violation.

The existing strconv.IntSize runtime skip does not help, checkptr fires inside unsafe.Slice itself, before any t.Skip could run.

What

• Move TestWriteFrameRejectsOversizedPayload out of frames_test.go into a new file frames_oversized_test.go carrying //go:build !race, so the file (and the unsafe.Slice call inside it) is excluded from the build under -race.
• Drop the now-unused strconv and unsafe imports from frames_test.go.

Verification

1. Non-race build still exercises the test.

   $ go test -run TestWriteFrameRejectsOversizedPayload -v ./pkg/execution/
   === RUN   TestWriteFrameRejectsOversizedPayload
   --- PASS: TestWriteFrameRejectsOversizedPayload (0.00s)
   PASS
   ok  	github.com/apache/airflow/go-sdk/pkg/execution	0.171s
   

2. Race build skips the test (no checkptr fatal).

   $ go test -race -run TestWriteFrameRejectsOversizedPayload -v ./pkg/execution/
   testing: warning: no tests to run
   PASS
   ok  	github.com/apache/airflow/go-sdk/pkg/execution	1.235s [no tests to run]
   

3. Whole package now passes under -race (previously fatal):

   $ go test -race ./pkg/execution/
   ok  	github.com/apache/airflow/go-sdk/pkg/execution	1.823s
   

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Was generative AI tooling used to co-author this PR?

• Yes, with help of Claude Code Opus 4.7 following the guidelines