Propagate @McpResource(annotations = ...) to MCP wire objects

[a-yeyang]

Fixes #6749.

Problem

@McpResource declares a nested annotations = @McpAnnotations(audience, priority, lastModified) element. Because Java annotation elements can never be null, the compiler always materialises the declared default, so the mere presence of @McpResource on a method makes annotations() return a non-null value regardless of whether the user actually set one.

None of the four resource providers observed today take that value into account:

• SyncMcpResourceProvider
• AsyncMcpResourceProvider
• SyncStatelessMcpResourceProvider
• AsyncStatelessMcpResourceProvider

Each of them builds McpSchema.Resource / McpSchema.ResourceTemplate directly and never calls Resource.Builder#annotations(...) / ResourceTemplate.Builder#annotations(...), so user-supplied audience / priority / lastModified values never reach the MCP client.

ResourceAdapter (the sibling helper) did look at annotations(), but it was gated on !lastModified.isEmpty(), silently dropped the lastModified value even when it fired, and did not handle the template path. It is also not used by the providers today, so its behaviour did not offset the provider-side omission.

Change

1. New mcp-annotations/common/ResourceAnnotationsUtils.toSchemaAnnotations(McpAnnotations):
   • resolves the declared default of McpResource#annotations() at class-load time via Method#getDefaultValue() — no default constants have to be duplicated;
   • returns null when the runtime value structurally equals that default (annotations implement structural equals), so the compiler-materialised default is not published to clients;
   • otherwise copies audience, priority and lastModified verbatim into an McpSchema.Annotations.
2. Call the helper from all four providers, for both the direct-resource and the URI-template paths, and set the resulting value via Resource.Builder#annotations(...) / ResourceTemplate.Builder#annotations(...).
3. Fix ResourceAdapter#asResource and #asResourceTemplate to use the same helper.
4. Add regression coverage across the four provider test classes:
   • default annotations() is not propagated;
   • explicit annotations reach McpSchema.Resource;
   • explicit annotations reach McpSchema.ResourceTemplate;
   • audience-only changes propagate for the sync stateful provider (proves conversion is not gated on lastModified).

Scope kept intentionally narrow

• The issue also mentions that lastModified cannot be dynamically computed per request; that is an API-shape change worth its own discussion and is not attempted here.
• Title propagation on the four provider paths is being handled separately in Propagate @McpResource title to generated resources #6787 for ResourceAdapter; this PR does not touch title so the two changes stay orthogonal.

Local validation

• mvn -pl mcp/mcp-annotations test → 1389 tests run, 0 failures, 0 errors, 2 skipped, including the new regression cases.
• mvn -pl mcp/mcp-annotations -DskipTests install (checkstyle + disable.checks profile) → BUILD SUCCESS.
• mvn -pl mcp/mcp-annotations spring-javaformat:validate -DskipTests → BUILD SUCCESS.

DCO

Signed-off-by.