feat: add excludeDefaultOptions for custom API compatibility

[Edison-A-N]

Add excludeDefaultOptions config for custom API compatibility

Fixes #5421
Fixes #6473
Related to #2661

Problem

Custom API providers fail when OpenCode automatically injects parameters they don't support:

From #5421:

Unsupported parameter: 'max_tokens' is not supported with this model.
Use 'max_completion_tokens' instead.

From #6473:

litellm.BadRequestError: invalid beta flag

These errors occur because OpenCode injects default parameters that custom APIs don't support:

• max_tokens / max_completion_tokens
• temperature, topP, topK
• thinkingConfig, reasoningEffort, promptCacheKey
• cache_control in messages

Why This Solution is Better

vs PR #5541

• #5541: Only fixes Azure reasoning models (hardcoded logic)
• This PR: Works for ANY custom API (config-driven)

vs #6473 Workarounds

• Workarounds: Require plugins or LiteLLM hacks
• This PR: Built-in config option, no external dependencies

Coverage

• ✅ Solves 3 related issues at once (#5421, #6473, #2661)
• ✅ 14 new unit tests
• ✅ JSDoc documentation
• ✅ Fully backward compatible

Solution

Add excludeDefaultOptions configuration at provider and model levels to disable all default parameter injection.

Config priority: model > provider > default (false)

Usage

{
  "provider": {
    "custom-api": {
      "excludeDefaultOptions": true,
      "npm": "@ai-sdk/openai-compatible",
      "options": { "baseURL": "...", "apiKey": "..." },
      "models": {
        "model-1": { "name": "Model 1" },
        "model-2": { "excludeDefaultOptions": false }
      }
    }
  }
}

What Gets Filtered

When excludeDefaultOptions: true:

• ❌ temperature, topP, topK, maxOutputTokens (unless explicitly in agent config)
• ❌ Provider options: thinkingConfig, reasoningEffort, promptCacheKey, cache_control, etc.
• ✅ Core parameters always sent: model, messages, tools, headers

Verification

• ✅ 144 tests pass (14 new tests added)
• ✅ Tested with custom API proxy - no more unsupported parameter errors
• ✅ Tested scenarios from #5421 and #6473
• ✅ Default behavior unchanged when not configured

Files Changed

8 files changed: +377, -6

• packages/opencode/src/config/config.ts - Config schema with JSDoc
• packages/opencode/src/provider/provider.ts - Config loading with priority
• packages/opencode/src/provider/transform.ts - Filter provider options
• packages/opencode/src/session/llm.ts - Filter core parameters
• packages/opencode/test/provider/provider.test.ts - 3 new tests
• packages/opencode/test/provider/transform.test.ts - 11 new tests
• packages/sdk/js/src/gen/types.gen.ts - SDK types
• packages/sdk/js/src/v2/gen/types.gen.ts - SDK v2 types