fix(github-copilot): preserve reasoning IDs for Copilot Codex models (#71684)

* fix(github-copilot): preserve all reasoning IDs and add gpt-5.3-codex support

The existing guard (8fd15ed0e5) only skipped rewriting reasoning item IDs
when encrypted_content was a non-null string. When gpt-5.3-codex is used
via GitHub Copilot, the model falls through to the forward-compat catch-all
with reasoning:false, so encrypted_content is never requested and arrives
as null — bypassing the guard and causing a rewrite. Copilot validates
reasoning item IDs server-side regardless of whether the client includes
encrypted_content, so the rewritten id triggers the 400 error.

Two changes:

1. connection-bound-ids.ts: skip ALL reasoning items unconditionally.
   Reasoning items always reference server-side state bound to their
   original ID; rewriting any of them breaks Copilot's lookup.

2. models.ts + index.ts: extend the forward-compat cloning logic to
   cover gpt-5.3-codex (adds it to the template-target set and to
   CODEX_TEMPLATE_MODEL_IDS so it can also serve as a template source
   for gpt-5.4). Adds gpt-5.3-codex to COPILOT_XHIGH_MODEL_IDS for
   the thinking profile.

Thanks @InvalidPandaa.

* docs(github-copilot): clarify gpt-5.3-codex is a no-op template for itself

https://claude.ai/code/session_01EAFmq4WyKkiUkVAqRXp4Bm

* fix(github-copilot): remove dead reasoning prefix branch in deriveReplacementId

https://claude.ai/code/session_01EAFmq4WyKkiUkVAqRXp4Bm

* fix(github-copilot): align reasoning id replay tests

* test(plugin-sdk): use cjs sidecar for require fast path

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/plugin-sdk/channel-entry-contract.test.ts

@@ -170,14 +170,14 @@ async function expectBuiltArtifactNodeRequireFastPath(
     fs.mkdirSync(pluginRoot, { recursive: true });
 
     const importerPath = path.join(pluginRoot, "index.js");
-    const sidecarPath = path.join(pluginRoot, "fast-path-sidecar.js");
+    const sidecarPath = path.join(pluginRoot, "fast-path-sidecar.cjs");
     fs.writeFileSync(importerPath, "export default {};\n", "utf8");
     // CommonJS so `nodeRequire` succeeds without falling back to jiti.
     fs.writeFileSync(sidecarPath, "module.exports = { sentinel: 7 };\n", "utf8");
 
     expect(
       channelEntryContract.loadBundledEntryExportSync<number>(pathToFileURL(importerPath).href, {
-        specifier: "./fast-path-sidecar.js",
+        specifier: "./fast-path-sidecar.cjs",
         exportName: "sentinel",
       }),
     ).toBe(7);