fix: preserve tools with dependency property-name maps - #1428

In a negotiated 2026-07-28 Streamable HTTP session, the client can remove a valid tool when its input schema has a property named x-mcp-header in dependentRequired or the legacy Draft 7 dependencies map. The annotation walker mistakes the property name for an off-chain header annotation, causing cache_tools_from_response to remove the tool from tools/list.

Handle the two keywords according to their contents:

• Skip dependentRequired during annotation traversal: it contains property names and arrays of names, not schemas.
• Walk the values of dependencies, ignoring the property names used as keys. Values can be name arrays or actual schemas; real schemas must still be checked for misplaced header annotations. This also prevents a dependency named default from concealing an invalid annotation.

For example, this schema is valid with either dependency keyword:

{
  "type": "object",
  "properties": {
    "x-mcp-header": { "type": "string" },
    "tenant": { "type": "string", "x-mcp-header": "Tenant" }
  },
  "dependentRequired": { "x-mcp-header": ["tenant"] }
}

Regression coverage verifies that name-array, schema, and boolean dependency values do not turn property names into annotations; valid tools remain in the returned list and cache; and actual off-chain annotations remain rejected, including inside dependency schemas named default, properties, or x-mcp-header.

References: JSON Schema 2020-12: dependentRequired, Draft 7: dependencies.

Local validation

The three new or expanded legacy-dependency regressions fail on the previous PR head 00f9202. With the legacy keyword handled as a map, all 59 header tests and 4 tool-list/cache tests pass.

Commands (one build job and one test thread):

cargo test -p rmcp --lib --features client,transport-streamable-http-client,transport-streamable-http-server transport::common::mcp_headers -- --test-threads=1
cargo test -p rmcp --lib --features client,transport-streamable-http-client,transport-streamable-http-server cache_tools_ -- --test-threads=1

The full workspace suite was not run locally.