(api-gateway) preserve PathPrefix when replacePrefixMatch is unset

[sujay-hashicorp]

Description

This change fixes the HTTPRoute PathPrefix regression where Envoy strips the matched prefix even when no rewrite is configured.

The regression came from the regex rewrite workaround added for replacePrefixMatch: "/". That logic also treated an empty/unset PrefixRewrite as a signal to strip the matched prefix, which changed the default behavior for plain PathPrefix routes.

With this change:

• PrefixRewrite == "/" still uses the regex workaround introduced for the // issue
• PrefixRewrite == "" does not emit any rewrite
• other non-empty rewrite values still use normal PrefixRewrite

This restores the original behavior for routes that only use PathPrefix: the full incoming request path is forwarded upstream unchanged.

Testing & Reproduction steps

Reproduction:

1. Configure an API Gateway HTTPRoute with a PathPrefix match such as /admin
2. Do not configure replacePrefixMatch / URLRewrite
3. Send a request like /admin/master/console
4. Before this change, the generated route emits regexRewrite and the upstream receives /master/console
5. After this change, no rewrite is emitted and the upstream receives /admin/master/console

Manual/behavior verification:

1. Confirm routes with no rewrite configured no longer emit regexRewrite
2. Confirm routes with PrefixRewrite == "/" still emit the regex-based rewrite workaround
3. Confirm routes with other non-empty rewrite values still emit standard PrefixRewrite

Tests run:

• GOFLAGS=-mod=mod go test ./agent/xds -count=1 -run TestUserRewriteLogic
• GOFLAGS=-mod=mod go test ./agent/xds -count=1 -run TestAllResourcesFromSnapshot

Relevant test updates:

• updated xDS rewrite logic unit test
• updated route goldens to remove unintended default regexRewrite output

Links

• Gateway API: Path rewrite using replacePrefixMatch results in // or no rewrite fix #23035

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.

  Revert plan: revert this PR to restore the previous rewrite behavior.

• If applicable, I've documented the impact of any changes to security controls.

  This change does not modify security controls.

[hc-github-team-consul-core]

📣 Hi @sujay-hashicorp! a backport is missing for this PR [23390] for versions [1.18,1.21] please perform the backport manually and add the following snippet to your backport PR description:

<details>
	<summary> Overview of commits </summary>
		- <<backport commit 1>>
		- <<backport commit 2>>
		...
</details>