[python] Add SDK version to REST User-Agent

[hutiefang76]

What is changed

Adds PyPaimon and Python version information to the default REST User-Agent. PVFS uses the standard header.User-Agent key and adds its PythonPVFS prefix without replacing an explicitly configured value.

The REST client now treats configured User-Agent keys case-insensitively, so header.user-agent is respected even when a request is prepared by requests. An explicit server override takes precedence over a client value, followed by server defaults and the SDK default. PVFS applies the same case-insensitive check before adding its own value.

Version metadata that cannot be read or parsed falls back to unknown, so requests continue normally.

Verification

• 45 tests passed across user_agent_test.py, build_info_test.py, rest/api_test.py, and pvfs_oss_filesystem_test.py, including prepared-request and precedence regressions.
• Flake8 passed for the changed files; changed Python files compiled successfully.

Closes #8543

[JingsongLi]

The linked #8543 is a concrete observability need, and the default version string now reaches real REST requests. I ran the four affected Python suites: 41 tests passed after allowing the local REST test server to bind its port. I also checked the prepared HTTP request, not only the in-memory header map.

[P2] Preserve explicitly configured User-Agent regardless of header casing. HTTP header names are case-insensitive, but base_headers.setdefault("User-Agent", ...) and PVFS's options.contains("header.User-Agent") are exact-case checks. With a valid configuration header.user-agent=custom-client/1.0, RESTApi keeps both user-agent: custom-client/1.0 and User-Agent: PyPaimon/...; requests.Session.prepare_request(...) sends the latter, silently overriding the explicit value. PVFS also inserts both keys. This violates the preservation requirement in #8543. Please detect existing User-Agent headers case-insensitively (also after REST config merge) and add a test that inspects a prepared request for a lowercase configured key.

Requirement fit: SUPPORTED. Requesting changes for the configuration contract before production use.

[hutiefang76]

Addressed the case-insensitive User-Agent feedback in ca90868bf. The new tests inspect the prepared HTTP request for lowercase client headers, including config merge/override behavior, and cover the PVFS option path.

The latest CI's flake8 check passes. Python Tests and Native CI still report the same two failures in interval_partition_test.py::test_signed_zero_key_ranges_keep_versions_in_one_split (SimpleNamespace lacks path_factory). That test file is unchanged by this PR and is identical on current origin/master; the PR changes only the User-Agent files. I am keeping this CI failure visible rather than treating the PR as green.

[hutiefang76]

Synced with master in d615576 to pick up the merged #10169 fix for the unrelated interval-partition CI failure. The targeted User-Agent, build-info, REST, PVFS and interval-partition tests pass locally (39 tests), and flake8 passes with the project config. The local Ray integration case was excluded because Ray is not installed. Fresh CI is now pending.