fix(logging): residual JSON fallback can still drop records after #1491

[groupthinking]

Summary

#1491 closed #1452 for circular containers and exploding __str__, but the recovery path can still raise. Three residual holes:

1. Exception whose own __str__ raises when formatting the fallback
2. Values forging __class__ = str that pass isinstance then fail in dump
3. Large ints past sys.get_int_max_str_digits() and non-finite floats (allow_nan=False)

Acceptance

• Record is never dropped for the cases above (or a final constant-record tier)
• Unit tests fail on unfixed code and pass with the fix
• Pre-existing CWE-117 / fix(logging): keep the record when JSON serialization fails (#1452) #1491 coverage still passes

Context

Implemented by #1515.

[linear-code]

GRV-405

[github-actions]

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[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• GRV-394

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