feat: add Grok 4.5 and Composer 2.5 Fast models

[fgnoah]

Summary

• Adds grok-4.5 and grok-composer-2.5-fast to DEFAULT_XAI_MODELS so the plugin auto-injects them into OpenCode's provider catalog.
• Updates the README model list / example opencode.json block / selection examples to match.
• Leaves the existing 4.3 / 4.20 model IDs in place.

Fixes #3

Notes

• Verified these model IDs work with SuperGrok OAuth against https://api.x.ai/v1/chat/completions.
• Intentionally scoped to just Grok 4.5 + Composer 2.5 Fast (not Grok Build aliases / other extras).

Test plan

• bun test (10 pass)
• bun run build
• After merge: rebuild plugin, restart OpenCode, confirm picker shows xai-oauth/grok-4.5 and xai-oauth/grok-composer-2.5-fast

Summary by CodeRabbit

• New Features

  • Added support for the grok-4.5 and grok-composer-2.5-fast models through xAI OAuth.
  • Updated default model options and configuration examples to highlight the newly supported models.

• Documentation

  • Refreshed setup instructions, troubleshooting guidance, and model references with the latest Grok model IDs.

• Tests

  • Expanded coverage to verify all supported default Grok models.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: d0e20ff6-f70a-46c7-9a38-18132430fd93

📥 Commits

Reviewing files that changed from the base of the PR and between 58487af and 4b161a6.

📒 Files selected for processing (3)

• README.md
• src/constants.ts
• test/plugin.test.ts

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📝 Walkthrough

Walkthrough

Changes

The xAI provider now defaults to grok-4.5 and grok-composer-2.5-fast while retaining grok-4.3. Tests verify all three identifiers, and README configuration examples and model references are updated.

Grok model support

Layer / File(s)	Summary
Model defaults and validation src/constants.ts, test/plugin.test.ts	Adds the two new default model identifiers and verifies them alongside grok-4.3.
Provider configuration documentation README.md	Updates model lists, configuration examples, selection instructions, reference entries, and troubleshooting examples for the new models.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: ysnock404

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	Title clearly summarizes the added Grok 4.5 and Composer 2.5 Fast model support.
Linked Issues check	✅ Passed	The PR adds the requested Grok 4.5 and Composer 2.5 models to the default catalog and docs.
Out of Scope Changes check	✅ Passed	All changes stay within model support, documentation, and tests, with no unrelated modifications.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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