chore(conformance): run against 0.2.0-alpha.12

[SergeevDmitry]

Bumps the conformance suite from 0.2.0-alpha.10 to 0.2.0-alpha.12 and adds the one client scenario the conformance client does not handle yet.

Motivation and Context

0.2.0-alpha.11 (2026-08-07) added the json-schema-2020-12-preservation client scenario (modelcontextprotocol/conformance#335). It is in the all suite for both 2025-11-25 and 2026-07-28. conformance-client has no arm for it and exits with Unsupported conformance scenario, so on alpha.11 or alpha.12 both client suites fail it:

[json-schema-2020-12-client-tool-found    ] FAILURE Client called tools/list and the mock server advertised 'json_schema_2020_12_tool'
[json-schema-2020-12-client-echo-completed] FAILURE Client called tools/call 'json_schema_echo' with a 'schema' object argument

The new arm does what the scenario asks:

1. connect with conformance_lifecycle();
2. list tools;
3. send the inputSchema of json_schema_2020_12_tool back through json_schema_echo as { "schema": ... }.

rmcp keeps every keyword the scenario checks: $schema, $defs, additionalProperties, allOf/anyOf, if/then/else, and $anchor.

How Has This Been Tested?

I ran every step of conformance.yml locally with npx @modelcontextprotocol/conformance@0.2.0-alpha.12, on macOS arm64 with stable Rust 1.99.

Step	main	This PR
server, 2025-11-25	83/83	83/83
server, 2026-07-28	153/153	153/153
server, the 10 task extension scenarios against expected-failures-extensions.yaml	exit 0	exit 0
client, 2025-11-25	208 passed, 2 failed	217/217
client, 2026-07-28	398 passed, 2 failed	407/407
client, extensions suite against expected-failures-extensions.yaml	not run	exit 0 (only the listed auth scenarios fail)

The server binary does not change in this PR, so its two columns are the same run.

• json-schema-2020-12-preservation alone: 9/9 on both spec versions.
• cargo clippy -p mcp-conformance --all-targets -- -D warnings is clean.
• cargo test -p mcp-conformance passes.
• expected-failures-extensions.yaml needs no change.

Breaking Changes

None. Only the conformance client and CI change.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

The arm returns an error if json_schema_2020_12_tool is not listed, so a change to the scenario shows up as a failed run instead of a silent pass.

AI assistance: I used Claude Code to help write this change.

[DaleSeo]

Thanks @SergeevDmitry! Left a minor comment.

[DaleSeo]

Thanks for the quick fix, LGTM.