fix(#3996): preserve Gemini user thinking budgets outside title generation

[aheritier]

What

Preserves Gemini user thinking budgets for ordinary image-output requests while retaining the special title-generation behavior, with guard and integration tests.

Why

Image-output requests must honor the user's configured reasoning budget; only title generation should use its constrained policy.

Validation

task test; Gemini client, modality, guard, and runtime/TUI integration tests.

Test instructions

Run:

task test

Issue a Gemini image-output request with a non-default user thinking budget and inspect the outgoing request. Expected: the configured budget is preserved. Trigger session title generation separately; expected: its constrained title-generation policy remains unchanged.

[aheritier]

Reviewed the three-dot delta vs workspace-media-naming (b18b517): 1 commit, 2 files (+70/-2). CI is green for 97877c1 (13 check-runs, incl. lint, build-and-test, CodeQL, windows-tests; publish jobs skipped). Locally: build + GOOS=windows build clean, go test -race green on pkg/model/provider/gemini, pkg/sessiontitle, pkg/model/provider, golangci-lint 0 issues, and the new test is load-bearing (reverting the production hunk fails title_generation_omits_thinking_config).

[blocking] Gemini 3 title generation loses the safeguard the comment still documents. client.go:455-458 returns before the Gemini-3 branch at :464-472, which set ThinkingLevelLow and raise MaxOutputTokens to a 200-token floor. Title generation clones with MaxTokens=20 (sessiontitle/generator.go:32,:148 -> clone.go:56-58), so the effective config for gemini-3-flash goes from {ThinkingLevel:LOW, MaxOutputTokens:200} to {ThinkingConfig:nil, MaxOutputTokens:20}. Per the comment retained at :460-463, Gemini 3+ models always think, so an omitted thinkingConfig means default thinking against a 20-token cap -> MAX_TOKENS with no visible text, which generateOnce reports as "empty title output" (generator.go:164-167) and Generate then burns every fallback (:135). Please narrow the omission to models that actually reject thinkingConfig (image-output models), or keep the MaxOutputTokens floor when omitting it, and add a title case asserting that floor.

[blocking] Title/body describe this change inversely. The only production change gates the existing NoThinking branch on GeneratingTitle(); the user-budget branch (:479-486) is untouched, so ordinary/image-output requests are byte-identical to base. The body claims the opposite ("preserves ... for ordinary image-output requests while retaining the special title-generation behavior") and lists modality/guard/runtime-TUI validation that is not in this delta. The commit body is accurate — please align title + body (and note that Compacting() is deliberately unaffected: compactor.go:202-206 never sets NoThinking).

[should-fix] return config at :457 also bypasses the structured-output block at :488-491 (verified: title+structured yields ResponseMIMEType "" vs "application/json" on the non-title path). Harmless today only because sessiontitle passes WithStructuredOutput(nil) (generator.go:147). Prefer scoping the condition: if c.ModelOptions.NoThinking() && !c.ModelOptions.GeneratingTitle() { ... }.

[should-fix] :428 doc comment says BuildConfig for the unexported buildConfig — and it's an unrelated drive-by edit in an otherwise focused diff.

[optional] The added cases use gemini-3-flash / gemini-2.5-flash; the motivating image-capable model (e.g. gemini-2.5-flash-image) isn't exercised, and nothing pins that ordinary image-model requests still carry the user's budget.

Checked and clean: no transport split (buildConfig has no apiSurface branching); ResponseModalities is applied after buildConfig (:763-768) so the #4020 guard is intact; the reranking caller (:881) overrides ThinkingConfig explicitly; no clone shared-state mutation (clone.go copies by value, -race clean); thinking_budget: 0 still disables thinking; modelerrors untouched. The pkg/mcp clientSession rename appears exactly once in the stack (9ffb3af, #4024 lane) — no duplication.