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>

extensions/github-copilot/models.test.ts

@@ -135,6 +135,50 @@ describe("resolveCopilotForwardCompatModel", () => {
     expect((result as unknown as Record<string, unknown>).reasoning).toBe(true);
   });
 
+  it("clones gpt-5.3-codex template for gpt-5.3-codex when not in registry", () => {
+    const template = {
+      id: "gpt-5.2-codex",
+      name: "gpt-5.2-codex",
+      provider: "github-copilot",
+      api: "openai-responses",
+      reasoning: true,
+      contextWindow: 200_000,
+    };
+    const ctx = createMockCtx("gpt-5.3-codex", {
+      "github-copilot/gpt-5.2-codex": template,
+    });
+    const result = requireResolvedModel(ctx);
+    expect(result.id).toBe("gpt-5.3-codex");
+    expect(result.name).toBe("gpt-5.3-codex");
+    expect((result as unknown as Record<string, unknown>).reasoning).toBe(true);
+  });
+
+  it("prefers gpt-5.3-codex as template source over gpt-5.2-codex for gpt-5.4", () => {
+    const template53 = {
+      id: "gpt-5.3-codex",
+      name: "gpt-5.3-codex",
+      provider: "github-copilot",
+      api: "openai-responses",
+      reasoning: true,
+      contextWindow: 300_000,
+    };
+    const template52 = {
+      id: "gpt-5.2-codex",
+      name: "gpt-5.2-codex",
+      provider: "github-copilot",
+      api: "openai-responses",
+      reasoning: true,
+      contextWindow: 200_000,
+    };
+    const ctx = createMockCtx("gpt-5.4", {
+      "github-copilot/gpt-5.3-codex": template53,
+      "github-copilot/gpt-5.2-codex": template52,
+    });
+    const result = requireResolvedModel(ctx);
+    expect(result.id).toBe("gpt-5.4");
+    expect((result as unknown as Record<string, unknown>).contextWindow).toBe(300_000);
+  });
+
   it("falls through to synthetic catch-all when codex template is missing", () => {
     const ctx = createMockCtx("gpt-5.4");
     const result = requireResolvedModel(ctx);
@@ -158,11 +202,20 @@ describe("resolveCopilotForwardCompatModel", () => {
     }
   });
 
+  it("infers reasoning=true for Codex model IDs", () => {
+    for (const id of ["gpt-5.4-codex", "gpt-5.5-codex", "gpt-5.4-codex-mini", "gpt-5.3-codex"]) {
+      const ctx = createMockCtx(id);
+      const result = requireResolvedModel(ctx);
+      expect((result as unknown as Record<string, unknown>).reasoning).toBe(true);
+    }
+  });
+
   it("sets reasoning=false for non-reasoning model IDs including mid-string o1/o3", () => {
     for (const id of [
       "gpt-5.4-mini",
       "claude-sonnet-4.6",
       "gpt-4o",
+      "mycodexmodel",
       "audio-o1-hd",
       "turbo-o3-voice",
     ]) {