fix: Remove stale 'no thinking blocks' comment; update anthropic-types with signature + richer usage

[HXYerror]

Part of #38. Small but important — fixes misleading documentation and missing types.

Background

non-stream-translation.ts:305 contains:

// GitHub Copilot doesn't generate thinking blocks, so we don't include them in responses

This comment is factually wrong as of Claude 4.5+ on the native Anthropic path. It was accurate for the OpenAI translation path (Copilot's /chat/completions doesn't return thinking), but confusing and misleading for anyone reading the code after #38 lands.

Additionally, anthropic-types.ts is missing fields that the native Anthropic path returns:

• AnthropicThinkingBlock.signature?: string — the encrypted thinking signature for multi-turn
• AnthropicMessagesResponse.usage.cache_creation.ephemeral_1h_input_tokens — returned by native path
• AnthropicMessagesResponse.usage.cache_creation.ephemeral_5m_input_tokens — returned by native path

Tasks

• In non-stream-translation.ts:305: replace the comment with:

  // Note: GitHub Copilot's /chat/completions endpoint does not return thinking blocks.
  // For Claude 4.5+ models, the native /v1/messages path (see create-messages-native.ts)
  // returns thinking blocks natively — no translation needed there.

• In anthropic-types.ts:
  • Add signature?: string to AnthropicThinkingBlock (or AnthropicThinkingContentBlock)
  • Add cache_creation?: { ephemeral_1h_input_tokens: number; ephemeral_5m_input_tokens: number } to the usage field in AnthropicMessagesResponse
  • Add inference_geo?: string to usage (returned by haiku model in live test)
• Search for any other hardcoded assumptions about Claude thinking that should be updated

Acceptance criteria

• No grep "doesn't generate thinking" hits in the repo after this lands
• TypeScript types correctly represent the native Anthropic response shape
• No test regressions

File pointers

• Touch: src/routes/messages/non-stream-translation.ts:305
• Touch: src/routes/messages/anthropic-types.ts

[HXYerror]

Implemented in this PR:

• anthropic-types.ts: Added signature? to AnthropicThinkingBlock for multi-turn reasoning round-trip
• AnthropicThinkingBlock: Added signature? field
• AnthropicMessagesPayload: Extended thinking to union type supporting both legacy { type: "enabled", budget_tokens } and new adaptive { type: "adaptive" }; added output_config.effort field for opus-4.7+
• AnthropicImageBlock: Added url source type alongside base64
• AnthropicToolResultBlock.content: Widened from string to string | Array<AnthropicTextBlock | AnthropicImageBlock>
• AnthropicResponse.usage: Documented service_tier as native-path field
• non-stream-translation.ts: Replaced stale comment; fixed image case to narrow source.type; added consola import

Closes #43