bugfix(plugins): Made compile_error! diag not include "s. - #10064

Summary

The compile_error! macro plugin now uses string_value(db) to extract the raw string content from the error message argument, stripping the surrounding quotes from the diagnostic output.

Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

⚠️ Note:
To keep maintainer workload sustainable, we generally do not accept PRs that
are only minor wording, grammar, formatting, or style changes.
Such PRs may be closed without detailed review.

Why is this change needed?

Error messages emitted by compile_error! were being displayed with their surrounding quotes included (e.g., "error message" instead of error message), which is inconsistent with how error messages are typically presented and mirrors the behavior of Rust's compile_error! macro.

What was the behavior or documentation before?

Diagnostics produced by compile_error!("error message") displayed the message as "error message" — with the quotes included as part of the diagnostic text.

What is the behavior or documentation after?

Diagnostics produced by compile_error!("error message") now display the message as error message, without surrounding quotes.

Related issue or discussion (if any)

N/A

Additional context

The change also adds a guard to handle the case where string_value(db) returns None, falling through to the existing error path for invalid arguments.