Hardens terminal background detection

[aboo]

What does this PR do?

Improves the robustness of terminal background mode detection by addressing several edge cases and potential failure points. This change aims to provide more reliable theme detection for the CLI.

• Prevents detection in non-TTY environments: The function now immediately returns 'dark' if stdout or stdin are not TTYs, ensuring correct behavior in non-interactive contexts like CI/CD pipelines where querying the terminal might lead to hangs or incorrect results.
• Enhances color format validation: Introduces strict validation for the detected background color, ensuring it matches a 6-digit hexadecimal format (#RRGGBB). Malformed or unsupported color formats (e.g., short hex #RGB, 8-digit hex #RRGGBBAA, or rgb() strings) will now correctly trigger a fallback to 'dark' mode, preventing potential parsing errors.
• Adjusts detection timeouts: Increases the timeout for both OSC (Operating System Command) support and color detection queries from 150ms to 300ms. This provides more leeway for various terminal emulators to respond, reducing false negatives due to slow responses.
• Adds comprehensive unit tests: The test suite has been significantly expanded to cover all new hardening conditions, including non-TTY scenarios, invalid color formats, OSC support failures, and a wider range of valid light and dark background colors.

Relates to PAP-7045