fix(ext/sw): consistent error detail across all 6 putBlob throw paths

The two GET steps (get-ref, get-commit) used resp.statusText, which is
often empty on HTTP/2. Now they read resp.text() like the other 4 throw
paths so every error message includes GitHub's response body for
debugging.

Plus a test assertion for calls[2] in the Git Data API path so a
transposition of GET ref / GET commit would be caught.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

extension/src/service-worker/__tests__/git-host.test.ts

@@ -55,6 +55,7 @@ describe('GitHubHost.putBlob', () => {
     expect(fetchMock).toHaveBeenCalledTimes(6);
     expect(fetchMock.mock.calls[0][0]).toContain('/git/blobs');
     expect(fetchMock.mock.calls[1][0]).toContain('/git/refs/heads/');
+    expect(fetchMock.mock.calls[2][0]).toContain('/git/commits/');
     expect(fetchMock.mock.calls[3][0]).toContain('/git/trees');
     expect(fetchMock.mock.calls[4][0]).toContain('/git/commits');
     expect(fetchMock.mock.calls[5][1]?.method).toBe('PATCH');