feat(core): add capability override layer

[Xuanwo]

Which issue does this PR close?

Refs #6707.

Rationale for this change

Some behavior test switches were encoded as service configuration even though they only control which capability-gated tests should run. This makes backend configuration mix real service behavior with test environment selection.

This PR adds a capability override layer so tests can disable selected capabilities without changing the backend's native capability declaration.

What changes are included in this PR?

• Add CapabilityOverrideLayer, which updates full_capability while preserving native_capability.
• Add OPENDAL_TEST_CAPABILITY_OVERRIDES support in testkit for boolean and numeric capability fields.
• Migrate S3 behavior test setup to capability overrides for versioning, append, and write-with-if-match test coverage.
• Deprecate S3 test-only capability toggles that are no longer needed.

Are there any user-facing changes?

S3 versioning, append, and write-with-if-match capabilities are now declared by default. The old S3 config toggles for these capabilities are deprecated and kept as no-op compatibility shims.

AI Usage Statement

AI assistance was used to prepare the implementation and validation steps.