Add structured output to the Rust weather server and client

Both tools declare an output_schema and return structured_content alongside
the text.

get_alerts declares Vec<Alert>, so it answers with a top-level JSON array
rather than an array nested in an object, which protocol revision 2026-07-28
is the first to allow. "No alerts" is simply []. get_forecast returns an
object, for contrast.

Error paths return a tool-level error: a tool declaring an output_schema MUST
return conforming structured content, so a path with no data has to fail.

The client compiles every declared outputSchema at connect time and validates
results against it. rmcp does not do this; its docs point at the jsonschema
crate, which is the one new dependency here. Each channel goes to its stated
reader: content is forwarded to the model, structured_content is used as data,
reporting how many items came back. A missing .env is no longer fatal, and the
API key is checked after connecting rather than before.

This supersedes #143, which bumped rmcp 0.3 -> 1.4 and failed CI because the
0.3 #[tool] macro API does not survive the jump. The rewrite was required
either way, so the bump is folded in and taken to 3.0.0-beta.2, the first
version with the 2026-07-28 model.

list_tools is hand-written. #[tool_handler] generates one hardcoding
ttl_ms: None, cache_scope: None, but both are required on a paginated result
at 2026-07-28 (SEP-2549) and a strict client rejects the response outright.
get_info does not pin ProtocolVersion::V_2026_07_28 either: rmcp's server
never implements server/discover, so pinning would overstate support.

Model identifier moves to claude-sonnet-5, replacing claude-sonnet-4-20250514,
which is past end of life.

Note that rmcp sends an array-rooted schema as written on every connection
rather than projecting it down, so an older client rejects the tool list.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

weather-server-rust/Cargo.toml

@@ -4,7 +4,7 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
-rmcp = { version = "0.3", features = ["server", "macros", "transport-io"] }
+rmcp = { version = "3.0.0-beta.2", features = ["server", "macros", "transport-io"] }
 tokio = { version = "1.46", features = ["full"] }
 reqwest = { version = "0.12", features = ["json"] }
 serde = { version = "1.0", features = ["derive"] }