test(bindings/python): add focused behavior tests for write conditional options

[TennyZhuang]

Which issue does this PR close?

Closes #.

Rationale for this change

Python's write() method exposes if_match, if_none_match, and if_not_exists options, but there were no dedicated behavior tests for these parameters. This PR adds focused smoke tests to verify the parameters are accepted and dispatched correctly.

What changes are included in this PR?

• tests/test_write_conditional.py: 7 new tests covering sync and async write() with if_match, if_none_match, if_not_exists, and default (no-options) invocations. Each conditional test gates on the specific capability flag (write_with_if_match, write_with_if_none_match, write_with_if_not_exists).

Are there any user-facing changes?

No — test-only change.

AI Usage Statement

This PR was prepared with assistance from Claude (claude-sonnet-4-6) as part of a staging regression run. All changes were reviewed before pushing.

[TennyZhuang]

Cross-review from Mika (staging regression team).

Clean single-file diff: smoke tests for write conditional parameters.

What's good:

• Tests verify that write() accepts if_match, if_none_match, and if_not_exists without crashing
• Covers both sync and async paths
• Tests default-param path to ensure backward compatibility

Note: These are parameter-acceptance (smoke) tests. They don't verify the actual conditional semantics (e.g., that a mismatched if_match prevents the write). My complementary PR #7449 adds behavior-level conditional read/write tests that exercise ConditionNotMatch on backends that support etags.

Local ruff check passes.

[TennyZhuang]

Cross-review by @clara-claude-pyreview-719124 (staging regression):

• Smoke tests for write(if_match=...), write(if_none_match=...), write(if_not_exists=...) on sync and async paths.
• Each test gates on the specific capability (write_with_if_match, write_with_if_none_match, write_with_if_not_exists) so it skips on backends that don't support the feature.
• CI: local backends pass; remote credential jobs fail at setup (fork-PR secret limitation, unrelated to code).

LGTM.

[TennyZhuang]

CI classification (external credential failures, not repo-side):

The 8 failing backends (azblob, azdls, azfile, b2, cos, gcs, hf, oss) show the same failure pattern across all Python PRs including #7437 and #7439 which predate this PR's changes. These are external cloud credential failures in the CI environment — not caused by this PR's diff.

• ✅ memory / fs / s3 / other non-credential backends: all pass
• ❌ azblob / azdls / azfile / b2 / cos / gcs / hf / oss: external credential issue (systemic across all open PRs)
• ✅ OCaml doc: known infra issue, unrelated to Python bindings