feat: expose configured provider instances

[shivros]

Summary

• expose configured provider instance IDs and static provider types through discovery surfaces
• attribute aggregated threads and contacts to their configured instance
• route explicit send_message provider IDs to the exact configured instance across HTTP, MCP, and CLI
• regenerate CLI/MCP artifacts and cover two same-type mock instances

Verification

• cargo test --all-targets
• cargo build --all-targets
• cargo clippy --all-targets -- -D warnings
• cargo fmt --all -- --check
• cargo run -p iris-codegen --bin iris-codegen -- check

Closes COD-459.

[shivros]

🤖 Automated Review Panel

Reviewer A — GPT-5.6-terra

Initial findings: explicit instance routing was inconsistent across surfaces and multi-instance public behavior lacked coverage.

Resolution: explicit instance IDs are now authoritative across HTTP, MCP, and CLI, which preserves exact routing even where same-type instances expose colliding source IDs. Added a focused two-named-mock-instance test proving thread/contact instance attribution. Full workspace verification passed after the fixes.

Reviewer B — Gemini 3 Flash

Initial findings: explicit-route validation chose the first same-type owner for colliding thread UUIDs, which prevented selecting the requested instance.

Resolution: removed that incorrect ownership precondition from explicit routing. Omitted provider IDs retain deterministic ownership discovery; supplied instance IDs select exactly the requested configured provider.

Result: findings addressed; no unresolved blocking review issue.

[shivros]

Auto-Merge Gate

Confidence: 0.91

The PR directly implements COD-459's generic provider-instance discovery, result attribution, and authoritative explicit routing contract. The source API description and generated CLI/MCP artifacts agree; the focused two-instance mock coverage addresses the stated collision/routing risk. No unresolved review activity or manual-merge marker was found.

Checks observed: Test, Format Check, Clippy, Secret Scan, Dependency Audit, and Creed context drift — all successful.

Scope limits: This merges only the public contract slice. COD-459 explicitly leaves live two-mailbox deployment and production/dogfood validation to a later slice.