test: Make Tool inputSchema tests exercise McpSchema.Tool

[imuromtsev]

Test-only follow-up to #749. After JsonSchema was replaced by Map<String, Object>, three tests in McpSchemaTests stopped checking what they were named for:

• testJsonSchema and testJsonSchemaWithDefinitions only round-trip a HashMap through Jackson, so no change to McpSchema can fail them. Their comments still say "Deserialize the original string to a JsonSchema object". They now deserialize {"name": …, "inputSchema": <schema>} into McpSchema.Tool and assert that the serialized $.inputSchema equals the original schema document, so $defs and definitions are checked end to end through the record.
• testToolWithMeta passed Map.of("inputSchema", schemaJson) as the input schema: a one-key map holding the whole schema as a string, which is not a schema. It now uses Tool.builder(name, JSON_MAPPER, schemaJson), like testTool and testToolWithComplexSchema.
• The outdated comments are removed, together with the now-unused TypeRef import.

No production code changes.

Verification

• ./mvnw -pl mcp-test -am -Dtest=McpSchemaTests test with -Pjackson2 and -Pjackson3 (JDK 21): 186 tests, 0 failures, 0 errors each; spring-javaformat:validate passes.
• Mutation check that the rewritten tests can fail (reverted, not in the commit):
  • renaming the inputSchema property in Tool.fromJson fails testJsonSchema and testJsonSchemaWithDefinitions (missing: $.inputSchema.$defs / $.inputSchema.definitions); before this change both passed under the same mutation, since they never touched Tool;
  • dropping meta in Tool.Builder.meta fails testToolWithMeta.

A draft-07 definitions schema deserializes into Tool without any validation error: the 2020-12 meta-schema check (SEP-1613) runs only when a server registers a tool, not during deserialization.

Prepared with an AI coding agent and reviewed by me.