fix(core): honor GOOGLE_GEMINI_BASE_URL and GOOGLE_VERTEX_BASE_URL

[chrisjcthomas]

Fixes #15430

Summary

This change wires custom base URL overrides through the core content generator so they are passed to @google/genai via httpOptions.baseUrl.

What Changed

• Added baseUrl to ContentGeneratorConfig
• Resolved GOOGLE_GEMINI_BASE_URL for AuthType.USE_GEMINI
• Resolved GOOGLE_VERTEX_BASE_URL for AuthType.USE_VERTEX_AI
• Passed baseUrl into GoogleGenAI through httpOptions
• Added validation so custom base URLs must be HTTPS unless they point to localhost
• Added unit tests covering Gemini and Vertex overrides, localhost HTTP overrides, invalid URLs, and insecure non-local HTTP URLs

Why

The Gemini CLI uses @google/genai, which supports a custom base URL via httpOptions.baseUrl. Before this change, the CLI propagated the environment variables but did not apply them when instantiating the SDK client.

Validation

• npm test -- src/core/contentGenerator.test.ts
• npm run typecheck

Notes

• I attempted the repo pre-commit hook as part of commit flow, but it currently fails due to an existing ESLint/plugin crash in import/no-internal-modules rather than this patch.
• The patch keeps localhost HTTP support for local proxies while requiring HTTPS for remote overrides.

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[chrisjcthomas]

Recreated on #25357 using a neutral branch name.