fix(compaction): dispatch summarization through the model runtime

[eddieparc]

Fixes #543

Problem

Compaction failed for every model served by a provider registered through pi.registerProvider() — the builtin claude-agent-sdk provider and extension providers such as senpi-accounts' Kiro:

Compaction rejected: compaction generator failed: No API provider registered for api: kiro-codewhisperer

compaction/speculative.ts dispatched the summarization request with compat's stream(), and compat's api-registry only knows the 10 builtin api ids (packages/ai/src/compat.ts BUILTIN_APIS); runtime-registered providers live solely in ModelRuntime / provider-composer. Normal agent turns worked because they dispatch through agent.streamFunction = modelRuntime.streamSimple, so a session on such a provider ran fine until it needed to compact — then it had no way out of the context window.

Change

Route the summarization stream through context.modelRegistry.modelRuntime.stream() when a registry is present, keeping the compat fallback for SpeculativeCompactionContext values constructed without one. SpeculativeCompactionContext already carried modelRegistry (it resolves summarization auth with it), so nothing new is threaded through. Same fix shape as #488 for /btw (ff48536).

Tested

• test/suite/regressions/543-compaction-runtime-provider.test.ts — new regression registering a provider whose api id exists only in ModelRuntime, then compacting. Captured RED before the fix with compaction generator failed: No API provider registered for api: runtime-only-compaction-api, GREEN after.
• npx vitest --run test/compaction test/suite/regressions — 604 passed. 4 pre-existing failures (0000-multi-session-theme-init, 2791-fswatch-error-crash, codemode-builtin-dedupe x2) reproduce identically on an unmodified origin/main checkout of the same worktree; unrelated to this change.
• npx tsgo --noEmit clean, biome check --error-on-warnings clean on the changed files, root npm run build exit 0.
• Real surface: local senpi built from this branch, running on the Kiro provider with a Claude model, /compact applies a summary instead of rejecting.

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Summary by cubic

Fixes compaction for runtime-registered providers by routing summarization through the model runtime, with compat as fallback. Adds support for header-auth providers and routes native remote compaction via the runtime. Fixes #543.

• Bug Fixes
  • Route summarization and remote-compaction streams via context.modelRegistry.modelRuntime when available; fallback to compat. Injected runners still win in openai-remote.ts.
  • Accept credential request headers as valid auth for summarization; apiKey no longer required if headers are provided.
  • Added regression tests: test/suite/regressions/543-compaction-runtime-provider.test.ts, test/suite/regressions/543-remote-compaction-runtime-provider.test.ts.

^{Written for commit 17b4eba. Summary will update on new commits.}

[eddieparc]

Pushed 17b4eba16: two more holes in the same class, so the fix is actually generic rather than kiro-shaped.

• Summarization no longer requires a resolved apiKey — a credential request header counts as resolved auth, so headers-authenticated providers (models.json headers, extension headers) can compact. The typed auth error for an unconfigured provider is unchanged.
• openai-remote.ts resolves its stream runner through the model runtime (resolveRemoteStreamRunner), so an extension openai-responses proxy that opts into compat.supportsRemoteCompactionV2 keeps its own transport on the native remote route. Injected dependencies.streamRunner still wins; compat is last.

New coverage: the header-auth case in 543-compaction-runtime-provider.test.ts and 543-remote-compaction-runtime-provider.test.ts, both captured RED before their fix. test/compaction + test/suite/regressions: 129 files / 610 tests passing. tsgo --noEmit, biome, and root npm run build clean. Detail in #543 (comment)