Validate message body ranges and harden receive JSON handling

[k-bx]

Summary

Prevent malformed Signal body ranges from being sent and ensure receive responses remain valid JSON when signal-cli emits logs or diagnostics on stderr.

Changes

• Validate mentions and text-style ranges against the final message’s UTF-16 length.
• Validate quote mentions against the quoted message.
• Reject quote mentions when no quote message is provided.
• Keep signal-cli stderr out of command responses.
• Preserve INFO and WARN log severity when classifying stderr.
• Treat unprefixed stderr diagnostics, such as stack traces, as warnings.
• Parse newline-delimited receive output as a JSON stream instead of constructing arrays through string concatenation.
• Return a clear error when receive output contains malformed JSON.
• Add regression coverage for Unicode offsets, invalid ranges, streamed messages, and stderr classification.

Motivation

Signal body-range offsets use UTF-16 code units. Offsets calculated using UTF-8 bytes—especially around emoji and other non-BMP characters—can produce messages that fail validation when received.

Additionally, combining signal-cli stdout and stderr could mix routine logs or exception output into JSON responses, making otherwise valid receive output unparsable.

Testing

go test ./...

[bbernhard]

Thanks for the PR!

Can you please do me a favor and split that up in smaller PRs - that makes it easier to review.