test: model OpenCode's real base headers in the plugin harness

[rynfar]

Problem

The V1 plugin harness returned only the plugin's chat.headers accumulator. OpenCode 1.18.11 builds the actual request in three stages:

1. native headers, including x-session-affinity and X-Session-Id;
2. model.headers;
3. plugin output, spread last.

Testing only stage 3 can make an identity change look effective when OpenCode's native affinity header still keeps the request keyed. PR #845 was the concrete example.

Changes

• Invoke the V1 hook with its real empty output object.
• Compose native, model, and plugin headers in actual wire order.
• Keep the non-Anthropic plugin-output assertion exact while proving native headers remain.
• Resolve final V1 headers through a real Hono request and pin the exact keys for build, title, summary, and compaction.
• Use the same real proxy boundary for existing V2 header rewrites:
  • title and summary resolve to no adapter key;
  • primary and compaction requests retain the base key;
  • ordinary subagents receive #agent scope.
• Remove the V2 fake-context Reflect.apply workaround.
• Add no production code and no forbidden casts or suppression directives.

Validation

Exact head: 38d7e6702a3903b971d20515a6e60c4a6255eef7

• npm test: 3,122 passed, 1 skipped, 0 failed
• focused V1/V2/session-key tests: 59 passed, 0 failed
• npm run typecheck
• npm run build
• npm pack --dry-run --json
• git diff --check origin/main...HEAD
• added-line audit: no as any, as unknown as, @ts-ignore, or @ts-expect-error

Two independent read-only reviews found no remaining blocker after the exact merge-stage and proxy-boundary repairs.