[MINOR] fix(client-python): initialize request headers for raw bodies

[yuqi1129]

What changes were proposed in this pull request?

Assign the merged request headers in the raw string/bytes body branch.

Why are the changes needed?

After #13743, this branch leaves request_headers uninitialized, causing UnboundLocalError and blocking main's Python CI with pylint E0606.

Failure: https://github.com/apache/gravitino/actions/runs/38882645940/job/116919740348

Does this PR introduce any user-facing change?

Restores requests with raw string/bytes bodies, including Ossie document imports. No API or configuration changes.

How was this patch tested?

• The existing raw-body regression fails before the fix (four string/bytes subcases) and passes afterward.
• All 1,239 Python unit tests passed.
• Pylint 4.0.5 for gravitino/utils/http_client.py and git diff --check passed.

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[github-actions]

Code Coverage Report

Overall Project	71.95% +0.25%	🟢
Files changed	85.54%	🟢

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[yuqi1129]

Close it as #13569 has already fixed it.