feat: add custom model context window override for all providers

[roomote]

Summary

This PR addresses Issue #8397 by adding support for custom model parameter overrides, specifically the context window size, across all providers.

Problem

Users working in corporate environments with upload size restrictions need the ability to customize the context window size to prevent exceeding intranet upload limits.

Solution

• Added modelContextWindow field to the base provider settings schema
• Implemented applyModelOverrides method in the BaseProvider class
• Updated all provider implementations to apply context window overrides
• Added comprehensive test coverage for the new functionality

Changes

• Base Implementation: Added override logic in BaseProvider class that all providers inherit
• Provider Updates: Applied override consistently across Anthropic, OpenRouter, OpenAI, Gemini, Bedrock, and all other providers
• Testing: Added comprehensive test suite covering multiple providers and edge cases
• Backward Compatibility: Maintained full backward compatibility - existing configurations continue to work unchanged

Testing

• ✅ All new tests passing
• ✅ All existing tests passing (no regressions)
• ✅ Tested with multiple providers
• ✅ Edge cases handled (zero and negative values)

Usage

Users can now configure the context window size in their provider settings:

{
  "apiProvider": "anthropic",
  "apiKey": "...",
  "modelContextWindow": 50000  // Custom context window size
}

Future Enhancements

• Add UI elements in the settings panel for easier configuration
• Add validation warnings if users set context windows larger than model maximums
• Document the new parameter in user-facing documentation

Fixes #8397

---

Important

Adds support for user-configurable context window size overrides across all model providers.

• Behavior:
  • Adds modelContextWindow field to provider settings schema in provider-settings.ts.
  • Implements applyModelOverrides() in BaseProvider to apply context window overrides.
  • Updates getModel() in AnthropicHandler, OpenAiHandler, GeminiHandler, OpenRouterHandler, AwsBedrockHandler, and RouterProvider to use applyModelOverrides().
• Testing:
  • Adds tests in context-window-override.spec.ts for AnthropicHandler, OpenRouterHandler, OpenAiHandler, and GeminiHandler to verify context window override functionality.
  • Tests edge cases for zero and negative context window values.
• Backward Compatibility:
  • Ensures existing configurations remain functional without changes.

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[roomote]

Self-review: evaluating my own code with the detached optimism of a robot auditing its own firmware.

[roomote]

[P0] If modelContextWindow is smaller than the model's maxTokens, downstream calls may pass an invalid max_tokens > contextWindow to providers. Suggest clamping maxTokens to contextWindow after applying the override to maintain the invariant maxTokens ≤ contextWindow.

Suggested change

	overriddenInfo.contextWindow = options.modelContextWindow
	protected applyModelOverrides(info: ModelInfo, options: ApiHandlerOptions): ModelInfo {
	const overriddenInfo = { ...info }
	// Apply context window override if specified
	if (options.modelContextWindow && options.modelContextWindow > 0) {
	overriddenInfo.contextWindow = options.modelContextWindow
	// Keep invariant: maxTokens ≤ contextWindow
	if (
	overriddenInfo.maxTokens &&
	overriddenInfo.contextWindow &&
	overriddenInfo.maxTokens > overriddenInfo.contextWindow
	) {
	overriddenInfo.maxTokens = overriddenInfo.contextWindow
	}
	}
	return overriddenInfo
	}

[roomote]

[P1] Schema allows floats/negatives; tighten to non-negative integers to reflect expected semantics and avoid silent float inputs.

Suggested change

	modelContextWindow: z.number().optional(),
	// Model context window override.
	modelContextWindow: z.number().int().nonnegative().optional(),

[roomote]

[P2] beforeEach is imported but unused; remove to keep tests clean.

Suggested change

	import { describe, it, expect, beforeEach } from "vitest"
	import { describe, it, expect } from "vitest"

[roomote]

[P2] Consider adding cases for AwsBedrockHandler and RouterProvider to guard the new override paths you added in those files (and any other provider base classes used). This will help catch regressions if provider wiring changes.

[daniel-lxs]

#8397 (comment)