skip invalid listener first

[zirain]

this's a follow up for #8361

Invalid earlier listeners can still win conflict resolution and block later valid listeners from being programmed. In listener.go (line 42) the new conflict passes run before the per-listener validation steps at listener.go (line 98) and listener.go (line 107). Then validate.go (line 710) and validate.go (line 781) use a first-wins policy without checking whether that earlier listener later becomes invalid. Reproduced locally with an invalid first HTTP listener (allowedRoutes.namespaces.from=Selector without a selector) followed by a valid TCP listener on the same port: the first listener ended up Programmed=False for Invalid, the second was forced to Accepted=False/Programmed=False for ProtocolConflict, and the IR had zero listeners on that port. So a malformed earlier listener can black-hole a later valid listener. The same ordering bug exists in the hostname-conflict path as well.

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[codecov]

Codecov Report

❌ Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.30%. Comparing base (ef8f744) to head (45bd0eb).

Files with missing lines	Patch %	Lines
internal/gatewayapi/validate.go	84.21%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8577      +/-   ##
==========================================
- Coverage   74.33%   74.30%   -0.04%     
==========================================
  Files         242      242              
  Lines       37793    37812      +19     
==========================================
+ Hits        28094    28096       +2     
- Misses       7751     7762      +11     
- Partials     1948     1954       +6     

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