Update http-client-python regen baseline to new azure-sdk-for-python branch/path (#10741)

[Azure/azure-sdk-for-python#46995](Azure/azure-sdk-for-python#46995)
moved the generated test baseline out of `main` into a dedicated branch
and folder. The regeneration script in `regenerate-common.ts` still
pointed at the old location, breaking baseline sparse-checkout.

### Changes
- `packages/http-client-python/eng/scripts/ci/regenerate-common.ts`:
  - `branch`: `main` → `typespec-python-generated-tests`
- `sourceSubdir`: `eng/tools/emitter/gen` →
`eng/tools/azure-sdk-tools/emitter/generated`
- Updated the matching doc comment on `prepareBaselineOfGeneratedCode`.
- Added a Chronus `internal` changelog entry for
`@typespec/http-client-python`.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: msyyc <70930885+msyyc@users.noreply.github.com>
Co-authored-by: Yuchao Yan <yuchaoyan@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

.chronus/changes/fix-regen-baseline-path-2026-5-20.md

@@ -0,0 +1,7 @@
+---
+changeKind: internal
+packages:
+  - "@typespec/http-client-python"
+---
+
+Update the baseline source for regeneration to use the `typespec-python-generated-tests` branch and `eng/tools/azure-sdk-tools/emitter/generated` path in `azure-sdk-for-python`, and enable `core.longpaths` on the temporary baseline clone to avoid Windows `MAX_PATH` errors during checkout.

packages/http-client-python/eng/scripts/ci/regenerate-common.ts

@@ -677,7 +677,7 @@ export async function preprocess(flavor: string, generatedFolder: string): Promi
 
 /**
  * Resets the `tests/generated/{azure,unbranded}` baseline by sparse-checking-out
- * `eng/tools/emitter/gen` from the Azure/azure-sdk-for-python repo, then
+ * `eng/tools/azure-sdk-tools/emitter/generated` from the Azure/azure-sdk-for-python repo, then
  * deleting a couple of fully-generated package folders so regeneration has to
  * recreate them from scratch (smoke test of full-emit path).
  *
@@ -686,8 +686,8 @@ export async function preprocess(flavor: string, generatedFolder: string): Promi
  */
 export async function prepareBaselineOfGeneratedCode(generatedFolder: string): Promise<void> {
   const repoUrl = "https://github.com/Azure/azure-sdk-for-python.git";
-  const branch = "main";
-  const sourceSubdir = "eng/tools/emitter/gen";
+  const branch = "typespec-python-generated-tests";
+  const sourceSubdir = "eng/tools/azure-sdk-tools/emitter/generated";
   const testsGeneratedDir = resolve(generatedFolder, "../tests/generated");
 
   console.log(pc.cyan(`\n${"=".repeat(60)}`));
@@ -708,6 +708,7 @@ export async function prepareBaselineOfGeneratedCode(generatedFolder: string): P
       execSync(cmd, { cwd: tempDir, stdio: ["ignore", "ignore", "inherit"] });
 
     run(`git init`);
+    run(`git config core.longpaths true`);
     run(`git remote add origin ${repoUrl}`);
     run(`git config core.sparseCheckout true`);
     run(`git sparse-checkout init --cone`);