feat(a2a): promote context IDs for classifier routing

[nerdalert]

Summary

Adds support for A2A’s standard contextId field, which groups related messages and tasks. Praxis now extracts contextId from request bodies and promotes it into safe internal routing metadata, so operators can route and observe A2A traffic by context when clients provide one. Requests without a context ID still pass normally. This PR does not store contextId -> backend ownership yet; it only makes context visible so a follow-up can add stateful context routing if needed.

This PR extracts context IDs from spec-defined request locations:

• SendMessage: params.message.contextId
• SendStreamingMessage: params.message.contextId
• ListTasks: params.contextId

When present and safe, the value is promoted to durable metadata, filter results, and the configured internal header, defaulting to x-praxis-a2a-context-id.

Scope

This is classifier metadata only. It enables configured router matches on contextId, but does not add automatic contextId backend ownership state, Valkey storage, or stateful context-based follow-up routing. The only thing intentionally not tested is stateful context ownership, because that is out of scope. This PR only promotes request metadata; it does not store contextId -> cluster mappings.

Validation

• cargo test -p praxis-ai-filters a2a
• cargo clippy -p praxis-ai-filters -- -D warnings
• make lint
• cargo +nightly fmt --all --check
• git diff --check

References

Refs #148
Refs #157
A2A spec: https://a2a-protocol.org/latest/specification/