feat: add base_url and custom_headers inputs for LLM Gateway integration

[GuitaristForEver]

Summary

Adds native base_url and custom_headers inputs to eliminate the need for env: block scripting when integrating with LLM Gateways and API management solutions.

Problem

Users in discussions #272, #575, and #704 are struggling to integrate with:

• LLM Gateways (Anthropic's recommended enterprise pattern)
• API proxies (Portkey, LiteLLM)
• API management (Azure APIM, AWS API Gateway)

The current workaround requires undiscoverable env: blocks and users don't know the JSON format for headers.

Solution

- uses: anthropics/claude-code-action@v1
  with:
    anthropic_api_key: ${{ secrets.API_KEY }}
    base_url: "https://your-gateway.example.com"
    custom_headers: '{"x-gateway-auth": "${{ secrets.TOKEN }}"}'

Changes

• action.yml - Add base_url and custom_headers inputs
• base-action/action.yml - Mirror inputs for standalone usage
• docs/configuration.md - Add LLM Gateway section with examples
• docs/cloud-providers.md - Reference new configuration
• base-action/README.md - Update inputs table
• base-action/test/validate-env.test.ts - Add test coverage (31 tests pass)

Test Plan

• YAML syntax verified
• All new tests pass (31 tests in validate-env.test.ts)
• CI checks will run on PR (typecheck, format, full test suite)
• Manual test with Portkey integration
• Manual test with custom base URL
• Verify backwards compatibility with env vars

Closes #840

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Note to reviewers: This unblocks users in #704, #575, and addresses the pattern requested in #272.