feat(api): per-request thinking effort override and adaptive effort envelope (DTE series 2/5)

[easonLiangWorldedtech]

Part of the DTE-v2 stack (≤400-line redo of Zoo-Code-Org#1338); this PR: adds per-request thinking-effort resolution (DTE-2) — the ADAPTIVE_OUTPUT_CONFIG_EFFORTS constant and the resolveEffectiveReasoningEffort precedence helper in src/api/transform/reasoning.ts, the transient reasoningEffort field on ApiHandlerCreateMessageMetadata in src/api/index.ts, and the accompanying spec.

Stack: base = feat/dte-v2-1-dynamic-thinking-effort (U1, PR #36, head 19788c03). Standalone diff vs that base:
113 additions + 1 deletion = 114 lines (≤400 budget). (measured with git diff --shortstat 19788c031667fc1e367511fb9026607652e0f9cb HEAD)

Related issue: #35 (DTE-v2 series tracking)

Amendments folded in (from plans/dte-gap-review.md): none for this unit.

Out of scope: the task-local runtime effort state and thinking-effort switch re-validation (U3), the persisted history / settings round-trip (U4), and the Anthropic adaptive output_config.effort envelope wiring that consumes these primitives (U5). This unit lands the resolution primitives and the API-surface metadata field only; no consumer reads metadata.reasoningEffort yet.

Pre-submission checklist:

• CI locally: pnpm check-types (11/11), targeted vitest (dte-effective-reasoning-effort.spec.ts 8/8), eslint --prune-suppressions --max-warnings=0 (exit 0; the three files carry no suppressions, so src/eslint-suppressions.json is untouched by this PR)
• i18n: not applicable — no locale files touched
• Tests accompany all changed lines: the spec covers every precedence branch of resolveEffectiveReasoningEffort (override > settings > model default, the disable sentinel, out-of-envelope override passthrough, undefined) and pins ADAPTIVE_OUTPUT_CONFIG_EFFORTS to the exact adaptive envelope set (mutation-gate corollary for the array literal)
• Visual baselines: none — no UI code in this unit
• Line budget measured vs stack base (number above)

Binary files in this PR: none

Content fidelity note (union extraction):

• Content source: union tree 27a2e97df (tag dte-legacy/union), legacy commit 6ea45b36a — "feat(task): task-local thinking effort state, per-request override, and adaptive effort envelope".
• src/api/transform/reasoning.ts (+45) and the new spec (+58) are taken whole-file from the union; the per-file union history confirms no later DTE unit modified them.
• src/api/index.ts is hunk-surgery: only the ReasoningEffortExtended import and the reasoningEffort metadata field (+9) are taken from the union. The union's removal of the case providerIdentifiers.geminiCli: fall-through line is a base-drift artifact (the union branch predates that upstream addition to main) and is deliberately not taken — the geminiCli case is preserved (the pre-commit prettier pass also normalized the indentation of that case's pre-existing comment by one tab — the PR's single deletion; the case line itself is unchanged).
• src/shared/__tests__/experiments.spec.ts (the ledger's 4th file for the legacy DTE-2a commit) needs no change here: it already matches the union state in the U1 base (empty diff vs 27a2e97df), so that ledger line is already contained in PR feat(settings): dynamic thinking effort experimental toggle (DTE-1) #36.

Verification (local, worktree wt-dte-v2-2, base feat/dte-v2-1-dynamic-thinking-effort @ 19788c031667fc1e367511fb9026607652e0f9cb):

• pnpm check-types → 11/11 green
• vitest: pnpm --dir src exec vitest run api/transform/__tests__/dte-effective-reasoning-effort.spec.ts → 1 file / 8 tests passed
• pnpm --dir src exec eslint --prune-suppressions --max-warnings=0 api/index.ts api/transform/reasoning.ts api/transform/__tests__/dte-effective-reasoning-effort.spec.ts → exit 0, no suppression entries in the three files
• git diff --shortstat 19788c031667fc1e367511fb9026607652e0f9cb HEAD → 3 files, 113+ / 1− = 114 lines

[easonLiangWorldedtech]

Superseded by upstream PR Zoo-Code-Org#1522 (DTE-v2 stack moved upstream — CodeRabbit only runs there). Same head 82358bd, same content.