Fix UNPREPARED handling in ResponseFuture to allow recovery on query ID mismatch

[Copilot]

The original PreparedQueryNotFound handler crashed with AssertionError when the server returned a query ID that differed from the local prepared_statement.query_id, permanently blocking reprepare/retry even when enough context existed to recover.

Root cause

# Original code — asserts IDs always match, but coordinator races/restarts can break this
if self.prepared_statement:
    query_id = self.prepared_statement.query_id
    assert query_id == response.info, "Got different query ID..."  # AssertionError → no recovery

Two additional defects in the same block:

• query_id.encode('hex') — Python 2 syntax, raises AttributeError in Python 3.
• Direct _prepared_statements[query_id] = ... — not thread-safe; bypasses _prepared_statement_lock.

Changes

• cassandra/cluster.py — Remove the assertion; use server-returned ID for cache lookup first, fall back to in-flight prepared_statement when the lookup misses; use add_prepared() for thread-safe cache writes; fix log calls to hexlify(...).decode('ascii') for clean string output.
• tests/unit/test_response_future.py — Add test_prepared_query_not_found_uses_local_prepared_context (regression: raises AssertionError on old code when server returns a different query ID) and test_prepared_query_not_found_prefers_returned_id_from_cache (validates cache-resolved statement takes precedence over local context). Docstring on the regression test explicitly documents the pre-fix failure mode.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

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