objstore: test accept-encoding-only GCS signing

[joechenrh]

What problem does this PR solve?

Issue Number: ref #69718

Problem Summary:

This is a draft E2E validation PR for the GCS S3-compatible import issue. The main proposed fix is #69721.

The specific question for this branch is whether excluding only Accept-Encoding from the SigV4 canonical request is enough for GCS S3 interoperability, while leaving the AWS SDK v2 metadata headers (Amz-Sdk-Invocation-Id and Amz-Sdk-Request) signed.

What changed and how does it work?

This PR keeps the GCS S3-compatible signer but changes it to remove only Accept-Encoding while signing.

It also adds draft-only HTTP fault injection for the GCS S3-compatible path so a real cluster E2E import can exercise AWS SDK retry behavior. The default behavior on this branch is:

• enabled only for GCS S3-compatible stores
• TIDB_GCS_S3_COMPAT_FAULT_RATE: default 0.02; set to 0 to disable
• TIDB_GCS_S3_COMPAT_FAULT_MODE: default mixed; allowed values are http500, connection-reset, and mixed
• TIDB_GCS_S3_COMPAT_FAULT_MAX: optional max number of injected faults per process

This PR is not intended to be the final mergeable fix as-is. It is intended to produce an image for online E2E validation.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Commands run locally:

make bazel_prepare
./tools/check/failpoint-go-test.sh pkg/objstore/s3store -run 'TestIsGCSS3Compatible|TestGCSS3CompatibleSignerSkipsAcceptEncodingWithRetry|TestGCSS3CompatibleFaultInjectionRetries' -count=1 -v
./tools/check/failpoint-go-test.sh pkg/objstore/s3store -run 'TestGCSS3CompatibleSignerSkipsAcceptEncodingWithRetry|TestGCSS3CompatibleFaultInjectionRetries' -race -count=1 -v
bazel test //pkg/objstore/s3store:s3store_test --test_sharding_strategy=disabled --test_arg=-test.run=TestIsGCSS3Compatible\|TestGCSS3CompatibleSignerSkipsAcceptEncodingWithRetry\|TestGCSS3CompatibleFaultInjectionRetries --test_output=streamed

Also attempted a failpoint-enabled Bazel run. The local Bazel build failed before tests with pkg/util/intest/assert_common.go:93:34: undefined: _curpkg_; failpoint state was disabled afterward.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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[APPROVALNOTIFIER] This PR is NOT APPROVED

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