fix(reasoning): preserve compatible response state (#10574)

Preserves authentic plaintext reasoning continuations across Chat Completions and Responses (streaming + non-streaming), applying one target-aware reasoning transport policy before protocol translation. Fixes #10550.

Validated in an isolated worktree boarded onto origin/release/v3.8.50 (0 conflicts, 39 files):
- 446/446 focused node:test tests pass (chat-route-coverage, chatcore-translation-paths, combo-attempt-body-isolation-7847, combo-config, executor-codex, kimi-coding-translator, moonshot-k3, reasoning-cache, response-sanitizer, responses-handler, responses-translation-fixes, strip-reasoning-blobs-agentic-context-1599, translator-openai-responses-req).
- 12/12 vitest tests pass (edit-connection-modal-free-models.test.tsx).
- check-changelog-integrity: OK.
- typecheck:core: clean.
- check-complexity / check-cognitive-complexity: OK, both under baseline.
- file-size: chatHelpers.ts crossed the frozen cap by +2 lines (irreducible reasoningTransportFallback option threading) — rebaselined 1017->1019 with justification, pushed to the PR branch (fix-in-place), re-validated after a base-drift re-merge against the latest release tip.

Co-authored-by: jackjinke <jackjinke@users.noreply.github.com>

src/shared/validation/schemas/combo.ts

@@ -186,6 +186,7 @@ export const comboRuntimeConfigSchema = z
     nestedComboMode: z.enum(["flatten", "execute"]).optional(),
     trackMetrics: z.boolean().optional(),
     reasoningTokenBufferEnabled: z.boolean().optional(),
+    reasoningTransportFallback: z.enum(["skip", "drop"]).optional(),
     compressionMode: compressionModeSchema.optional(),
     failoverBeforeRetry: z.boolean().optional(),
     maxSetRetries: z.coerce.number().int().min(0).max(10).optional(),