build(deps): consolidate seven Dependabot PRs

[araujof]

Summary

• Upgrade direct sha2 consumers to 0.11.0 and preserve the Python host's SHA-256 hex output.
• Upgrade cel to 0.14.0. Update the custom-function fixture for the new built-in overload dispatch and document policy migration notes.
• Remove the direct rand 0.8 test dependency. RSA 0.9 requires rand_core 0.6, so the proposed rand 0.9 bump is incompatible; tests now use rsa::rand_core::OsRng.
• Upgrade actions/setup-python to v7, actions/setup-go to v7, and actions/deploy-pages to v5.
• Use dtolnay/rust-toolchain@stable with an explicit toolchain: "1.96.0" in every Rust workflow. The proposed 1.120.0 compiler is unreleased and made all Rust jobs in build(deps): bump dtolnay/rust-toolchain from 1.96.0 to 1.120.0 #176 fail. This keeps the action current without changing the project's Rust MSRV.
• Refresh rustls to 0.23.45 for RUSTSEC-2026-0285 and replace the yanked chacha20 0.10.1 with 0.10.2 so the supply-chain gate passes.
• Keep the Python host e2e test environment's resolved mcp version when the checked-out framework imports MCPError; older checkouts that import McpError still get the compatibility downgrade.

Dependabot PRs superseded

• build(deps): bump dtolnay/rust-toolchain from 1.96.0 to 1.120.0 #176 — Rust toolchain action (resolved with @stable and explicit compiler pin)
• build(deps): bump actions/setup-python from 5 to 7 #161 — actions/setup-python 5 → 7
• build(deps): bump actions/setup-go from 5 to 7 #125 — actions/setup-go 5 → 7
• build(deps): bump sha2 from 0.10.9 to 0.11.0 #103 — sha2 0.10.9 → 0.11.0
• build(deps): bump cel from 0.13.0 to 0.14.0 #102 — cel 0.13.0 → 0.14.0
• build(deps): bump rand from 0.8.6 to 0.9.4 #101 — rand 0.8.6 → 0.9.4 (resolved by removing the direct dependency)
• build(deps): bump actions/deploy-pages from 4 to 5 #99 — actions/deploy-pages 4 → 5

Checks

• make lint
• make audit
• cargo test --workspace --locked --offline -q
• CARGO_NET_OFFLINE=true make examples-build
• Parsed every GitHub Actions workflow as YAML