feat: support custom Anthropic API baseUrl and apiKey

[sunyuhan1998]

Summary

• Add agent-client.claude.api-key and agent-client.claude.base-url configuration properties for enterprise users to route requests through internal API gateways
• These properties translate to ANTHROPIC_API_KEY and ANTHROPIC_BASE_URL environment variables in the CLI process, with explicit properties taking precedence over the env map
• Fix ClaudeAgentOptions.environmentVariables not being propagated to the CLI process (dead code)

Closes #25

Changes

File	Change
ClaudeAgentProperties.java	Add apiKey/baseUrl fields, update buildCLIOptions() to merge into env map
ClaudeAgentOptions.java	Add apiKey/baseUrl fields and builder methods
ClaudeAgentModel.java	Translate environmentVariables/apiKey/baseUrl into CLIOptions.env
ClaudeAgentAutoConfiguration.java	Pass apiKey/baseUrl/env from properties to options builder
Tests	13 new tests covering defaults, setting, precedence, and coexistence

Configuration Example

agent-client:
  claude:
    api-key: "${ANTHROPIC_API_KEY}"
    base-url: "https://proxy.example.com"

Precedence

Explicit apiKey/baseUrl properties > env map entries > system ANTHROPIC_API_KEY

Test Plan

• Unit tests: ClaudeAgentPropertiesTest (7 new tests), ClaudeAgentModelTest (6 new tests)
• Integration test verified against third-party Anthropic-compatible endpoint
• Formatting clean: spring-javaformat:validate passes
• All 65 existing tests pass

[sunyuhan1998]

Hi @markpollack, would you mind taking a look at this PR when you get a chance? This addresses issue #25 which was previously attempted in #26 but became stale after the SDK rewrite. Thanks!