fix: reject non-finite withdrawal amounts

[iamdinhthuan]

Summary

• reject NaN and infinity withdrawal amounts before nonce/database work
• keep malformed JSON and invalid amount validation covered by Flask client tests
• make the withdrawal validation tests import the dotted integrated-node filename through importlib and share the module to avoid duplicate Prometheus metric registration

Fixes #4420

Verification

• python3 -B -m pytest -q node/tests/test_withdraw_amount_validation.py node/tests/test_withdrawal_validation.py --tb=short -p no:cacheprovider
• python3 -B -m py_compile node/rustchain_v2_integrated_v2.2.1_rip200.py node/tests/test_withdraw_amount_validation.py node/tests/test_withdrawal_validation.py
• git diff --check

[jaxint]

LGTM! Great work on this PR. 🚀

[TJCurnutte]

Approved. The non-finite withdrawal amount guard is in the right place: immediately after float(raw_amount) and before nonce/database work.

What I checked:

• math.isfinite(amount) rejects NaN, inf, and -inf before balance, nonce, or signature logic can operate on non-finite floats.
• Existing malformed JSON, nonnumeric amount, negative amount, and minimum-withdrawal behavior remains covered by the focused tests.
• The shared import name in the tests avoids duplicate module/Prometheus registration during the combined test run.

Validation performed on head 9b3dc1a5ef2ab87eb3371cc423408220f879e67f:

• git diff --check origin/main...HEAD -- node/rustchain_v2_integrated_v2.2.1_rip200.py node/tests/test_withdraw_amount_validation.py node/tests/test_withdrawal_validation.py passed.
• /Users/curnutte/.hermes/hermes-agent/venv/bin/python3 -B -m py_compile node/rustchain_v2_integrated_v2.2.1_rip200.py node/tests/test_withdraw_amount_validation.py node/tests/test_withdrawal_validation.py passed.
• /Users/curnutte/.hermes/hermes-agent/venv/bin/python3 -B -m pytest -q node/tests/test_withdraw_amount_validation.py node/tests/test_withdrawal_validation.py --tb=short -p no:cacheprovider returned 12 passed in 0.24s.
• Temp-DB Flask probe compared origin/main with this PR for /withdraw/request: origin/main let NaN/inf reach database work and returned 500 in the stripped temp DB path, while this PR returned HTTP 400 with Amount must be a finite positive number for NaN, inf, and -inf before DB access.

I do not see a blocking issue in this diff.

[Scottcjn]

Closing as duplicate of #5719 (william08190, first-poster — filed 4 hours earlier at 11:07 UTC vs 15:52 UTC here, both reference issue #4420 'reject non-finite withdrawal amounts'). #5719 was merged with credit. Thanks for the parallel work; per the first-poster-per-issue rule, credit goes to whoever lands first.