docs(google): fix incorrect mode enums and outdated limitations in Google provider docs

[mimran-khan]

Summary

I ran into the same confusion described in #2289 while setting up Instructor with Google GenAI. The docs had several issues that made it genuinely hard to figure out which modes to use. Here's what I fixed:

Mode enum confusion - The backwards-compatibility notes in both genai.md and google.md listed Mode.TOOLS and Mode.JSON as both the deprecated modes and the recommended replacements, which makes no sense. The actual deprecated modes are Mode.GENAI_TOOLS, Mode.GENAI_JSON, and Mode.GENAI_STRUCTURED_OUTPUTS. Fixed to match what the code actually does.

Optional types work fine - genai.md warned that "Gemini doesn't have support for Union and Optional types", but Optional (X | None) works correctly. Only true Union types (e.g., str | int) are unsupported.

Streaming warning contradicted itself - It said Mode.JSON doesn't support streaming, then in the same section showed working streaming examples with Mode.JSON. Clarified that Mode.TOOLS doesn't stream, but Mode.JSON does.

Other fixes:

• Removed outdated Enum limitation (enums work fine, verify_no_unions is a no-op)
• Fixed a Python syntax error in the Vertex AI migration example (misplaced parenthesis)
• Removed "Coming soon" for Gemini Flash-8b which has been available for over a year
• Updated patching.md to use generic Mode.TOOLS instead of deprecated provider-specific mode names

Verification

Cross-referenced all changes against the v2 source:

• instructor/v2/providers/genai/handlers.py - registers Mode.TOOLS and Mode.JSON
• instructor/v2/core/mode.py - DEPRECATED_TO_CORE map confirms GENAI_TOOLS → TOOLS, etc.
• instructor/v2/providers/gemini/utils.py - verify_no_unions() always returns True

Partially addresses #2289

Checklist before requesting a review

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests.
• If it is a core feature, I have added documentation.

[jxnl]

Reviewed the docs changes. This is the best current Google docs PR for #2289: it fixes the incorrect legacy mode names, narrows the Union/Optional guidance, removes the stale enum limitation, clarifies streaming mode behavior, and fixes the Vertex example syntax without over-expanding the migration story. Good to merge after maintainer merge approval.

[jxnl]

Selected this as the Google docs PR to carry #2289 forward. I closed narrower duplicate #2329 and left #2343 open only as a possible narrower follow-up for provider-prefix / Vertex migration wording after this lands.

[jxnl]

Consolidated these Google/GenAI/Vertex docs corrections into draft PR #2400. Closing this PR so the reviewed docs fixes land through the weekly maintenance consolidation.