fix(pymongo): decode bytes values instead of str()-ing them to avoid BytesWarning (#4782)

[DawnofGenX]

What

Under python -b (BytesWarning mode), the PyMongo integration emits BytesWarning: str() on a bytes instance — see #4782. The flagged line (data["operation_ids"]["session"] = str(lsid) in 2.37.0, str(lsid_id) on master) stringifies the BSON logical session id, which is a bson.binary.Binary — a bytes subclass. Two stringify sites in CommandTracer.started() have the same problem:

1. str(lsid_id) → session id becomes the literal string "b'...'" (plus BytesWarning under -b).
2. json.dumps(command, default=str) → any raw bytes value in the command (e.g. a bytes collection name, as reported) goes through str() the same way.

How

• Add _bytes_safe_str(): value.decode("utf-8", errors="replace") for bytes (and subclasses like Binary), plain str() otherwise. Undecodable bytes get U+FFFD replacements instead of an exception; nothing is caught or hidden — the actual stringification is fixed.
• Use it at both sites above.

Tests

New regression tests in tests/integrations/pymongo/test_pymongo.py:

• test_bytes_safe_str — helper unit test (decode, invalid-UTF-8 fallback, non-bytes passthrough).
• test_bytes_lsid_does_not_raise_byteswarning (PII on/off) — drives CommandTracer.started() with a Binary lsid under warnings.simplefilter("error", BytesWarning) and asserts operation_ids.session is a clean decoded str, no b' prefix.
• test_bytes_collection_name_in_query_does_not_raise_byteswarning — same with a bytes collection name in the command; asserts the span description contains test_collection, not b'test_collection'.

Verified locally (py3.12, pymongo 4.18.1, mockupdb):

• All 4 new tests fail on master (BytesWarning raised as error / b'...' in description) and pass with the fix.
• Full directory: pytest tests/integrations/pymongo/ → 55 passed (51 existing + 4 new), also under python -b with 0 BytesWarning lines in output.
• ruff format / ruff check clean.

Per the discussion in the issue (decode vs. repr): this goes with decoding, since the b'...' form was clearly not intended for span data.

Fixes #4782

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[DawnofGenX]

Good catch — fixed in 3ef7ab5. The lsid is now rendered as a lossless hex string via a dedicated _bytes_to_hex() helper (Binary`` bytes → .hex()) instead of UTF-8-decoding random bytes, so distinct session ids stay distinct and no BytesWarning is emitted. The UTF-8 decode remains only for human-readable BSON values like collection names (the json.dumps(default=...) site), where decoding is the right call. The regression test now uses a real 16-byte Binary UUID and asserts the span's operation_ids.session equals the exact hex of the id.