fix(payloads): SayText2 overflow

[Rushaway]

Description

This PR fixes a runtime crash in SourceChatRelay when relayed chat/event messages exceed SayText2 byte limits after color/prefix formatting.

Background

Production logs showed exceptions from MoreColors at EndMessage with:

• “SayText2 message content …”
• “This message exceeded SayText2's maximum bytes allowed”

The failure happened in the receive path before broadcasting with CPrintToChatAll.

Root cause

The final rendered message length (prefix + colors + user text) could exceed SayText2 payload capacity. Existing sanitization removed unsupported multibyte characters, but did not enforce a strict byte budget on the final outgoing payload. #6

What changed

1. Added a conservative payload cap constant
2. Added UTF-8-safe byte clamping helper
3. Added prefix-aware payload clamp helper
4. Applied clamping in chat receive flow before broadcast
5. Applied clamping in event receive flow before broadcast:

Behavior after fix

• Messages that fit are unchanged.
• Oversized messages are truncated safely (without breaking UTF-8 character boundaries).
• Truncated content gets an ellipsis.
• No SayText2 overflow exception is expected from this path.

Risk and compatibility

• Low risk: change is scoped to inbound relay display formatting.
• No protocol changes and no API changes.
• Slightly more aggressive truncation is possible due to conservative safety margin, by design.

Testing notes

• Verified static diagnostics: no editor-reported errors in the modified file.
• Runtime expectation: reproducing the previous long-message case now results in a truncated broadcast instead of an exception.