fix: preserve incoming Anthropic beta headers

[mkazlauskas]

Summary

Preserve SDK-provided anthropic-beta flags when rewriting OAuth requests, alongside the bridge defaults and required Claude Code/OAuth flags.

Problem

buildBetaFlags(existing) currently ignores existing. This can leave beta-gated fields in the request body while removing the SDK header that enables them. The reported failure on OpenCode 1.18.31 with bridge 1.10.12 was:

thinking.adaptive.block_binding: Extra inputs are not permitted

The patched OpenCode Anthropic SDK adds a thinking-binding-controls beta for this field. Preserving incoming flags avoids losing that capability declaration. This does not establish that every model or deployment supports the field.

Changes

• Merge incoming flags with configured bridge flags.
• Keep the Claude Code and OAuth flags first.
• Trim whitespace, remove empty entries, and deduplicate all flags.
• Add three regression tests against the production helper, including a synthetic SDK feature flag.

The separate plugin startup failure (modelId.match is not a function) is already fixed on upstream main by the { id, server } default export, with existing regression coverage.

Validation

• npm test: 125 tests passed.
• npm run typecheck: passed.
• git diff --check: passed.
• Manual testing with Opus 5.5 passed

[olosegres]

I also use rhis code locally in my fork, @dotCipher please consider to merge it