Make updater configuration immutable

[Harshil-1603]

Description of the changes being introduced by the pull request:

Make UpdaterConfig a frozen dataclass and expose Updater.config as a read-only property. This prevents changing settings after initialization or refresh, when some settings have already been used to initialize trusted metadata or the default fetcher.

This intentionally changes the API: callers that assign settings on an existing configuration must pass them to UpdaterConfig(...) instead, or use dataclasses.replace(...) to derive settings for a new Updater. The API docstrings explain that migration. Existing limit and download tests now pass their settings at construction and retain their behavioral assertions.

A regression test covers both mutation through the caller's original config reference and mutation/replacement through the updater, before and after refresh. It fails on the previous implementation.

Fixes #2471

Validation:

• Full unittest suite: 206 tests passed.
• Coverage: 97%; required 97% gate passed.
• Ruff checks and formatting: passed.
• Full-tree mypy in the isolated lint environment: 49 source files passed.
• Sphinx HTML documentation build with warnings treated as errors: passed.
• Zizmor workflow checks: passed.

AI assistance: Codex GPT-6 was used to prepare the change.