ci: declare MSRV and add MSRV check

[DaleSeo]

Summary

The project sets edition = "2024" (which requires Rust >= 1.85) but never declared a Minimum Supported Rust Version, so the MSRV was implicit and untested. Downstream consumers had no machine-readable floor, and nothing prevented a change from silently raising it.

This PR:

• Declares rust-version = "1.88" on [workspace.package] in the root Cargo.toml, and inherits it (rust-version = { workspace = true }) in the published rmcp and rmcp-macros crates so the value actually applies to what's published to crates.io.
• Adds an MSRV Check CI job that enforces the promise on every push/PR.

Why 1.88?

Edition 2024 sets a hard floor of 1.85. Compiling upward from there in this lockfile, the floor is raised to 1.88 by current dependencies that declare rust-version = "1.88.0" — notably darling, jsonwebtoken, and time. Toolchains 1.85–1.87 fail at the resolver stage; 1.88 is the lowest version where both rmcp (all features except local) and rmcp-macros compile cleanly, so it is the true MSRV.

The toolchain-override pitfall (handled)

The repo's rust-toolchain.toml pins a newer dev toolchain, and that file overrides both rustup default and the toolchain installed by dtolnay/rust-toolchain@<ver> for in-repo cargo invocations. A naive MSRV job would therefore silently test the pinned toolchain instead of the MSRV. To avoid that, the job sets RUSTUP_TOOLCHAIN: "1.88" on the run step (which takes precedence over rust-toolchain.toml) and prints rustc --version so the effective toolchain is visible in logs.

The job stays intentionally minimal (checkout, install toolchain, Swatinem/rust-cache@v2, cargo check) — cargo check for rmcp/rmcp-macros needs no Node/uv/Python setup.

Test plan

• cargo +1.88 check -p rmcp --features <all-except-local> and cargo +1.88 check -p rmcp-macros both compile cleanly.
• Confirmed 1.85/1.86/1.87 fail (dependency rust-version requirements force 1.88).
• Verified cargo metadata reports rust_version=1.88 for both rmcp and rmcp-macros (inheritance works).
• Verified that with rust-toolchain.toml present, RUSTUP_TOOLCHAIN=1.88 makes rustc report 1.88.0 (job uses the MSRV, not the pinned toolchain).
• YAML validated with yaml.safe_load.