Update Rust dependencies for pair relay

[wesbillman]

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Summary

• refresh the root Rust lockfile and migrate sprout-pair-relay to secp256k1 0.31
• replace the removed rand-std feature with rand + std, and update Schnorr sign/verify call sites for the 0.31 API
• make pair-relay connection shutdown send an explicit WebSocket Close frame
• update MCP #[tool_handler] usage required by the refreshed rmcp macro behavior

Builds on the now-merged #523 by handling the Rust pair-relay migration that was intentionally left out of that dependency batch.

Tests

• cargo test -p sprout-pair-relay
• cargo clippy --workspace --all-targets -- -D warnings
• just test-unit
• pre-push hook: rust/web/desktop/mobile checks, builds, and tests passed