Replace security-critical assert statements with explicit exceptions

[ChihweiLHBird]

• Replace assert with ValueError in headers.py for proxy count validation and app.py for duplicate blueprint detection
• assert statements are stripped when Python runs with -O, silently disabling these checks in production

[Copilot]

Pull request overview

This PR replaces security-sensitive assert checks (which are stripped under Python -O) with explicit exceptions to ensure misconfiguration and invalid state are still enforced in production.

Changes:

• Replace an assert on PROXIES_COUNT with a ValueError in parse_xforwarded
• Replace an assert on duplicate blueprint registration with a ValueError in Sanic.blueprint

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
sanic/headers.py	Converts proxy-count validation from assert to an explicit exception in forwarded-header parsing.
sanic/app.py	Converts duplicate-blueprint detection from assert to an explicit exception during blueprint registration.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

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[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.710%. Comparing base (785d77f) to head (87e5992).

Files with missing lines	Patch %	Lines
sanic/headers.py	0.000%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #3149       +/-   ##
=============================================
- Coverage   87.793%   87.710%   -0.083%     
=============================================
  Files          105       105               
  Lines         8143      8145        +2     
  Branches      1290      1292        +2     
=============================================
- Hits          7149      7144        -5     
- Misses         687       693        +6     
- Partials       307       308        +1     

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