Sxian/clt 2940/improve error messages for tlsca certificate failures in region

[xianshijing-lk]

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PR description

1. Improved error chain preservation - Added error_with_chain() helper that walks the full std::error::Error::source() chain, so errors now show:
   failed to retrieve region info: error sending request for url (...): error trying to connect: invalid peer certificate: UnknownIssuer
2. Added documentation - Added doc comments to SignalError::RegionError explaining common causes and fixes (install ca-certificates or use rustls-tls-webpki-roots feature)
3. Added TLS feature documentation in Cargo.toml - Documented the TLS features and highlighted rustls-tls-webpki-roots as the recommended option for container deployments
4. Added unit tests - 8 new tests verifying the error chain is properly preserved and surfaced

Tests:
cargo test -p livekit-api --features signal-client-tokio region

Breaking changes

N/A

MSRV

N/A

Testing

cargo test -p livekit-api --features signal-client-tokio region

Async

n/A

[github-actions]

Changeset

The following package versions will be affected by this PR:

Package	Bump
livekit-api	patch