ci: add MSRV and feature matrix checks

[harsh04044]

Adds two new CI jobs and fixes two bugs caught while testing them locally.

New jobs:

• MSRV - builds and tests on Rust 1.88 (the actual minimum, verified against all deps)
• Feature matrix - tests --no-default-features, default, and --all-features combinations

Bugs fixed:

• rust-version was declared as "1.70" but the real minimum is 1.88 due to dependencies (rmcp uses edition 2024, darling requires 1.88)
• e2e_happy_path test was missing required-features = ["rmcp", "test-utils"] in Cargo.toml, causing default feature builds to fail