fix(sdk-go): delete uploaded extension on validation failure (#2725)

[teddiesloco]

Summary

• Fixes Go SDK leaks an uploaded Browserbase extension when upload-response validation fails #2725
• In packages/sdk-go/browserbase_session.go, if extension.validate() fails after a successful upload, invoke deleteExtensionBestEffort using context.WithoutCancel(ctx) when a non-empty extension ID is present.
• Prevents accumulating orphaned extensions on the Browserbase account when metadata validation fails on otherwise successful uploads.

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Summary by cubic

Fixes orphaned Browserbase extensions accumulating when extension metadata validation fails after a successful upload.

• The SDK now deletes the uploaded extension best-effort on validation failure when a non-empty extension ID is present.
• Cleanup uses context.WithoutCancel(ctx), so deletion still runs even if the original request context is canceled.
• The original validation error is still returned, joined with any cleanup error via errors.Join.

^{Written for commit 1d9f004. Summary will update on new commits.}

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