Playground: Refactor messages types and parsing (string enums, readonly)

[monokkai]

Summary

Playground: Refactor message types and parsing for stronger typing and robustness
Convert numeric enums to string enums to make runtime values explicit.
Make Message fields readonly and codeframe optional to express data intent.
Normalize message parsing to handle CRLF and trailing whitespace and to treat empty bodies as no codeframe
Confine changes to the playground to reduce risk to core packages.

How did you test this change?

Manual smoke test (playground UI): start the playground and exercise message flows:
Send a single-line message → expect title populated and no codeframe
Send a multi-line message with trailing spaces → expect title to be first line (trimmed) and codeframe to contain trimmed remaining lines.
Send a CRLF message (\r\n) → expect codeframe normalized to LF (\n).
Verify UI: icon matches level, codeframe displays in the message body when present, and dismiss button behavior (non-Playground sources dismissible) remains unchanged.