Fix trailing slash dropped when joining route segments

[Copilot]

Explicit trailing / on interface routes was being dropped when joined with child route segments starting with ? or :.

@route("{blobName}/")
interface Blobs {
  @put @route("?some-query=true")
  putBlob(blobName: string, @body content: bytes): void;
  // Expected:  /{blobName}/?some-query=true
  // Actual:    /{blobName}?some-query=true
}

Root cause: normalizeFragment unconditionally trimmed trailing / from all non-last segments, even when the next segment doesn't start with /.

Fix: Replace the blanket trimLast behavior with // deduplication at the join point — only strip a / when concatenation would produce a double slash.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Regression from http 1.10.1</issue_title>
<issue_description>Explicit / is now dropped when defined on interface.
When joining with parent routes we seem to drop an explicit trailing /. this feels quite like an edge case but this isn't expected

playground

@route("{blobName}/")
interface Blobs {
  // expected:  /{blobName}/?some-query=true
  // actual:  /{blobName}?some-query=true
  @put
  @route("?some-query=true")
  putBlob(blobName: string, @body content: bytes): void; 
}
```</issue_description>

<agent_instructions>The route joining algorithm should be smarter and not trim `/` if it is relevent. Only trim it if it would result in a `//` </agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Regression from http 1.10.1 #10064

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