feat: harden anthropic provider path

[gay00ung]

Summary

This PR hardens the native Anthropic Messages API path for d2t.

Included changes

• add config coverage for provider = "anthropic"
• verify the default Anthropic resolution path through TOML loading
• add a native request smoke test for:
  • POST /v1/messages
  • x-api-key
  • anthropic-version: 2023-06-01
• add provider-specific usage docs for Anthropic

Why this change

d2t already had Anthropic support in code, but the provider path was still missing clearer validation around:

• default config behavior
• request shape expectations
• user-facing setup guidance

This PR makes the Anthropic path more explicit and easier to trust.

Validation

• ./gradlew --no-daemon :apps:cli:test --tests dev.diff2test.android.cli.D2tConfigTest
• ./gradlew --no-daemon :modules:test-generator:test --tests dev.diff2test.android.testgenerator.OpenAiResponsesTestGeneratorTest

Notes

• this PR focuses on the native Anthropic provider path only
• qwen/custom provider behavior is not changed here