fix(provider): meta muse spark 1.1 provider

[gonzoga]

Summary

Add Meta as a new built-in provider to ForgeCode, using the Meta Model API at https://api.meta.ai/v1. Meta's API is OpenAI Responses-compatible with Bearer token authentication via MODEL_API_KEY.

Context

Meta recently launched their Model API with Muse Spark 1.1, a multimodal model supporting agentic tool calling, coding, structured output, image/video understanding, and long-context reasoning with a 1M-token context window. The API is drop-in compatible with the OpenAI Responses API, making this a pure configuration-driven integration with no new behavioral code needed.

Changes

• crates/forge_repo/src/provider/provider.json: Add meta provider entry with OpenAIResponses response type, MODEL_API_KEY env var, and hardcoded muse-spark-1.1 model (1,048,576 context, tools + parallel tools + reasoning, text + image input)
• crates/forge_domain/src/provider.rs: Register META constant, add to built_in_providers(), add from_str() and display_name() match arms, add 3 new tests + update 2 existing tests
• crates/forge_repo/src/provider/provider_repo.rs: Add test_meta_config test verifying provider config, response type, URL, and hardcoded model specs
• README.md: Add Meta env var documentation entry

Testing

• cargo check passes with no errors
• cargo test -p forge_domain -- provider::tests — 42 passed, 0 failed (including 3 new Meta tests)
• cargo test -p forge_repo -- provider_repo::tests — 14 passed, 0 failed (including test_meta_config)
• cargo insta test --accept — no snapshot changes needed

Notes

• Uses OpenAIResponses response type per Meta's recommendation for "the full feature set for agentic workflows" including reasoning replay across turns
• The InputModality enum only supports Text and Image — the Meta API also accepts video and PDF inputs, but these cannot be represented in the current domain model
• Follows the Vivgrid precedent (OpenAIResponses + hardcoded models) and the Neuralwatt precedent (hardcoded model content verification)

Co-Authored-By: ForgeCode noreply@forgecode.dev

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