fix: merged policy status

[kkk777-7]

What this PR does / why we need it:
Fix arko's comment.

• I'm pinpointing a medium-severity bug where merged routes get overwritten instead of appended when both listener-specific and gateway-level merges exist—leading to inaccurate status reporting. I'm
  crafting a detailed comment referencing internal/gatewayapi/helpers.go:1187, explaining how replacing the assignment with an append fixes the issue. After careful reasoning about listener keys,
  section names, and merge logic, I see this as a correctness regression worth flagging while confirming no other major regressions appear.

> - internal/gatewayapi/helpers.go:1187 — Medium — When sectionName != nil we populate mergedRoutes with the listener-specific entries, but the fallback that also checks the gateway-level key
  (SectionName: "") assigns into the same slice again. If both map entries exist (e.g. a listener-scoped merge plus a gateway-scoped merge coming from SectionName-less parent refs), the second
  assignment overwrites the first list. That drops the listener-level merges from the merged message and they'll leak back into the "overridden" condition. Please append instead of overwrite:

    mergedRoutes = append(mergedRoutes, routeSet.UnsortedList()...)

Fix Description

• Overwriting mergedRoutes with the entry for gwKey
  • Since the merge logic is implemented on a per-listener basis, the gwKey entry will never be populated in practice.
    • https://github.com/envoyproxy/gateway/blob/main/internal/gatewayapi/backendtrafficpolicy.go#L315-L319
  • Therefore, this redundant code handling gwKey has been removed.
• Duplicate routes are being included in the status.
  • Changed the logic to deduplicate routes that are merged multiple times across different listeners.

Related PR

#6888

Which issue(s) this PR fixes:

Fixes #

Release Notes: No

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.32%. Comparing base (689d266) to head (b872a25).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7376      +/-   ##
==========================================
- Coverage   72.39%   72.32%   -0.07%     
==========================================
  Files         231      231              
  Lines       33896    33892       -4     
==========================================
- Hits        24538    24513      -25     
- Misses       7601     7619      +18     
- Partials     1757     1760       +3     

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