fix(handler): generate a tool's outputSchema for the serialize contract - #1378

@atarutin this is mergeable at b1b6189. It fixes the example in #1374, and reverting the one line in schema_for_output fails the new test.

non-blocking: this changes what existing servers advertise. required grows on upgrade for a plain Option field and a #[serde(default)] field. A client that generates types from outputSchema sees those fields flip from optional to required. That is the intended correction, and the PR title carries it into the changelog.

non-blocking: #[tool(output_schema = schema_for_type::<T>())], as in test_json_schema_detection.rs:59, stays on the deserialize contract. That stays untouched, since an explicit override is the author saying what they mean.

$ cargo test -p rmcp --features client,server,macros,elicitation --lib --test test_json_schema_detection --test test_list_tools_result --test test_structured_output --test test_tool_builder_methods   # head b1b6189, rustc 1.96.1
241 passed; 7 passed; 1 passed; 21 passed; 6 passed
# schema_for_output calling schema_for_type again, new tests kept
schema_for_output_describes_the_serialized_form ... FAILED

# struct { name: String, note: Option<String>, #[serde(default)] tags: Vec<String> }, schema_for_output::<T>()
# base 50fba5c   "required":["name"]
# head b1b6189   "required":["name","note","tags"]    # "tags" still carries "default":[]
$ cargo fmt --check   # clean

# not run: clippy, the full --all-features suite (no pkg-config for openssl-sys here)

@connor4312, you're the assigned reviewer. The changelog line covers the required change. Does it also need a release note?

I read the diff at this head (code-read only, no local run). The direction looks right: tool outputs are serialized by the server, so outputSchema should describe the serialize form, and splitting the serialize cache (CACHE_FOR_SERIALIZED) from the deserialize cache means a type requested both ways cannot get the wrong schema from the other cache. I also confirmed the #[tool] macro path calls schema_for_output (rmcp-macros/src/tool.rs), so generated tools pick this up.

One gap between the docs and the tests: the doc comment on schema_for_serialized promises two more serialize-contract behaviors, a Decimal serializing as a string and a #[serde(default)] field always being written, but the new tests only cover Option with and without skip_serializing_if. Either add cases for those two, or narrow the comment to what the tests cover.

One scoping question: this fixes attribute-driven divergence between the two forms. A type with a hand-written Serialize impl that writes a different shape than its JsonSchema derive describes would still mismatch, since schemars works from the derive. Is that intentionally out of scope here, or worth a note in the function docs?

thanks!