feat: expose HTTP response headers on non-streaming responses

[bborn]

Summary

Fixes #110

• Adds _status and _headers accessors to BaseModel, making HTTP response metadata available on non-streaming API responses
• Modifies base_client.rb to set these accessors after coercing the response model
• Uses respond_to? guard so non-BaseModel return types are unaffected

This mirrors the existing pattern on streaming responses (which already expose .status and .headers via BaseStream).

Usage

response = client.messages.create(
  model: "claude-sonnet-4-20250514",
  max_tokens: 1024,
  messages: [{ role: "user", content: "Hello" }]
)

response._status  #=> 200
response._headers #=> {"anthropic-ratelimit-requests-remaining" => "4", "request-id" => "req_abc123", ...}

Test plan

• Added test/anthropic/resources/messages/response_headers_test.rb with tests for header and status access on non-streaming responses
• Verified existing streaming tests still pass
• Verified existing type/model tests still pass

🤖 Generated with Claude Code