fix: use raw model names from models/list in model picker fix #430

[Dimillian]

Motivation

• Align the UI with backend-provided model names so displayed labels match the raw values returned by models/list instead of a local title-casing/slug formatter.
• Users observed a mismatch (e.g. gpt-5.3-codex vs GPT-5.3-Codex) that could be misleading or cause inconsistencies across clients.

Description

• Stop transforming model slugs for display and instead use the server-provided displayName/display_name when present and non-empty, otherwise fall back to the raw model slug in parseModelListResponse in src/features/models/utils/modelListResponse.ts.
• Removed the previous slug-formatting usage from the model-list parsing flow.
• Updated unit tests in src/features/models/utils/modelListResponse.test.ts to assert the new behavior (preserve explicit displayName and use raw slugs when displayName is missing or empty).

Testing

• Ran the focused unit tests with npm run test -- src/features/models/utils/modelListResponse.test.ts and they all passed.
• Ran type checking with npm run typecheck and it completed successfully.

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Codex Task