Fix UnboundLocalError when OSM header has non-numeric sequence number

[xovishnukosuri]

Summary

• get_replication_header() in src/osmium/replication/utils.py crashes with UnboundLocalError when an OSM file header contains a non-numeric osmosis_replication_sequence_number value.
• The except ValueError handler logged seq instead of seqstr. Since seq is only assigned inside the try block and the assignment fails for non-numeric strings, seq is unbound at the point the warning is emitted.
• Fix: replace seq with seqstr so the raw invalid string is reported, matching the intent of the warning message.
• Two new tests cover the invalid-sequence and negative-sequence warning paths, both of which were previously untested.

Root cause

# before (line 60 in utils.py)
except ValueError:
    LOG.warning("Sequence id '%s' in OSM file header is not a number. Ignored.", seq)
    #                                                                              ^^^
    # seq is unbound here — UnboundLocalError at runtime

# after
except ValueError:
    LOG.warning("Sequence id '%s' in OSM file header is not a number. Ignored.", seqstr)
    #                                                                              ^^^^^^
    # seqstr holds the raw value that failed int() conversion

Test plan

• test_get_replication_header_invalid_sequence — verifies the function returns sequence=None and logs the bad string without raising
• test_get_replication_header_negative_sequence — verifies the existing negative-value path also works and logs correctly
• All 359 existing tests continue to pass

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