fix(conformance-test): HTTPRoutePathRewrite

[AlinsRan]

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches
• Documentation
• Refactor
• Chore
• CI/CD or Tests

What this PR does / why we need it:

fix two issues:

1. Prefix-only paths do not match
   Requests like /prefix do not match the pattern because it requires a trailing /.

2. Extra or missing slashes
   The replacement template unconditionally appends /$2, causing incorrect results when $2 is empty or when replaceTarget is /.

  rules:
  - matches:
    - path:
        type: PathPrefix
        value: /prefix/one
    filters:
    - type: URLRewrite
      urlRewrite:
        path:
          type: ReplacePrefixMatch
          replacePrefixMatch: /one
  - matches:
    - path:
        type: PathPrefix
        value: /strip-prefix
    filters:
    - type: URLRewrite
      urlRewrite:
        path:
          type: ReplacePrefixMatch
          replacePrefixMatch: /

request path	expect	actual
/strip-prefix	/	unmatched
/prefix/one/two	/onet/wo	/onetwo

Request path | Expected path | Actual path -- | -- | -- /strip-prefix | / | unmatched /prefix/one/two | /one/two | /onetwo

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[ronething]

lgtm