fix: validate effective chat picker readiness

[itsJai42]

Summary

• validate pre-send readiness against the current Chat picker's full {provider, model, baseUrl} identity
• keep credential inspection in the main process
• skip Desktop-local .env and auth.json credential probes for Remote and SSH connections while preserving missing-model validation
• document the model and credential ownership boundaries in lat.md

Why

Chat already reran readiness when picker state changed, but IPC validated only the persisted global model. That produced NO_ACTIVE_MODEL for valid session picks. Forwarding the picker identity exposes a second boundary: Remote and SSH credentials live on the other host, so Desktop cannot infer MISSING_API_KEY from local files.

This supersedes #839, credits its picker-override approach, and incorporates the Remote/SSH refinement reported in #839 (comment).

Verification

• RED: 3 focused regressions failed against current main with NO_ACTIVE_MODEL, MISSING_API_KEY, and missing model-detection mismatch
• GREEN: focused validation/preload/IPC-contract suite: 226 passed
• full suite: 1,747 passed, 3 skipped
• npm run typecheck: passed
• npm run build: passed
• changed-file ESLint: 0 errors
• Prettier, git diff --check, and lat check: passed

Existing baseline

Full npm run lint remains nonzero because current main has 9 errors in unchanged src/renderer/src/components/ProviderKeysSection.tsx. This PR adds no lint errors.

[greptile-apps]

Greptile Summary

This PR makes Chat readiness follow the conversation’s effective model selection. The main changes are:

• Pass the picker’s provider, model, and base URL through preload IPC.
• Validate the effective session model instead of only persisted configuration.
• Skip Desktop-local credential probes for Remote and SSH connections.
• Add focused validation tests and document credential ownership.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
src/renderer/src/screens/Chat/Chat.tsx	Passes the current conversation’s effective routing identity into readiness validation.
src/preload/index.ts	Forwards the optional model override through the preload IPC bridge.
src/preload/index.d.ts	Updates the renderer-facing API declaration for the model override.
src/main/ipc/register.ts	Derives credential-check scope from the current connection mode.
src/main/validation.ts	Validates the supplied session model and conditionally checks local credentials.
tests/validation.test.ts	Covers session model selection, remote credential scope, and missing-model validation.
lat.md/model-selection.md	Documents readiness routing and local versus remote credential ownership.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant Chat as Chat renderer
    participant Preload as Preload API
    participant Main as Main IPC handler
    participant Validation as Readiness validation
    Chat->>Preload: validate(profile, effective model identity)
    Preload->>Main: validate-chat-readiness
    Main->>Main: Read connection mode
    Main->>Validation: Validate model and credential scope
    alt Effective model is empty
        Validation-->>Chat: NO_ACTIVE_MODEL
    else Local connection
        Validation->>Validation: Check local credentials
        Validation-->>Chat: Readiness result
    else Remote or SSH
        Validation->>Validation: Skip Desktop credential probes
        Validation-->>Chat: Ready
    end

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%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
sequenceDiagram
    participant Chat as Chat renderer
    participant Preload as Preload API
    participant Main as Main IPC handler
    participant Validation as Readiness validation
    Chat->>Preload: validate(profile, effective model identity)
    Preload->>Main: validate-chat-readiness
    Main->>Main: Read connection mode
    Main->>Validation: Validate model and credential scope
    alt Effective model is empty
        Validation-->>Chat: NO_ACTIVE_MODEL
    else Local connection
        Validation->>Validation: Check local credentials
        Validation-->>Chat: Readiness result
    else Remote or SSH
        Validation->>Validation: Skip Desktop credential probes
        Validation-->>Chat: Ready
    end

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