chore(deps): update A2A, MCP, and supporting crates

[danielkov]

Summary

Update the exact dependency pins for A2A, MCP, JSON Schema validation, keyring, terminal images, and TOML. This brings in fractional multipleOf validation fixes, terminal-image detection and Sixel rendering fixes, and MCP HTTP transport improvements.

Impact

• RMCP now defaults to 16 concurrent ordinary HTTP POSTs, with a separate control slot and 5-second control/session-recovery timeouts. Cancellation, ordering, recovery, and discovery behavior also change.
• Terminal-detection fallback can enable images where Kit previously fell back to text.
• A2A 0.11 changes REST/gRPC error mappings and push-webhook content type; Kit's current JSON-RPC transport path is unaffected.

Technical details

Dependency	Update
a2a-protocol-client, a2a-protocol-server, a2a-protocol-types	0.10.0 → 0.11.0
jsonschema	0.52.0 → 0.52.1
keyring	4.1.6 → 4.2.0
ratatui-image	11.0.6 → 11.0.8
rmcp	3.1.4 → 3.2.0
toml	1.1.4 → 1.1.5, retaining spec-1.1.0

The lockfile also updates the required jsonschema-regex, jsonschema-value, and referencing companions to 0.52.1. The new jsonschema-value dependency on zmij reuses its existing locked version.

Preserve all existing feature selections and leave upstream-constrained generic-array, matchit, smallvec, and the thiserror family unchanged. Exclude unrelated Windows dependency deduplication from the lockfile.

[kit-code-agent]

No issues found. The changes look good to merge.