feat: add Bedrock API Key auth and dynamic model fetching

[zw2497]

Summary

• Bedrock API Key authentication: Add support for the new AWS Bedrock API Key auth method (announced July 2025) as an alternative to IAM credentials. Users can switch between API Key and IAM auth via a segmented control in settings.
• Dynamic model fetching: Implement listModels() using AWS Bedrock ListFoundationModels + ListInferenceProfiles APIs, enabling automatic model discovery instead of relying solely on hardcoded defaults.
• Both auth methods (API Key and IAM) are supported for model listing via aws4fetch SigV4 signing (IAM) or Bearer token (API Key).

Changes

• src/shared/providers/definitions/models/bedrock.ts — Add API Key auth to provider, implement listModels() with foundation model capability mapping
• src/shared/providers/definitions/bedrock.ts — Pass effectiveApiKey to model constructor
• src/renderer/routes/settings/provider/$providerId.tsx — Add auth mode toggle UI (API Key vs IAM)
• src/shared/types/settings.ts — Add 'iam' to activeAuthMode enum

Test plan

• Configure Bedrock with API Key auth → verify chat works
• Configure Bedrock with IAM credentials → verify chat works
• Switch between auth modes in settings → verify UI updates correctly
• Click "Check" button with valid credentials → verify model list is fetched
• Verify fetched models show correct capabilities (vision, tool_use)

🤖 Generated with Claude Code

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