fix: serial number stored in DB now matches certificate serial number

[devangpratap]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Closes #121

Description of Changes

_generate() built the certificate using int(self.serial_number) but never read
the serial number back after signing. The DB value came from _generate_serial_number()
while the certificate value came from what the signing library actually embedded. In the
pyOpenSSL path set_serial_number() silently truncated 128-bit UUID integers during
signing, producing two genuinely different integers — exactly what was reported in #121.

Fix: after cert = builder.sign(...), sync the field back:
`self.serial_number = str(cert.serial_number)

Added test_serial_number_db_matches_certificate to verify DB and certificate serial
always match. All 74 existing tests continue to pass.

Screenshot

N/A

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📥 Commits

Reviewing files that changed from the base of the PR and between 9056c37 and 39ee622.

📒 Files selected for processing (2)

• django_x509/base/models.py
• django_x509/tests/test_cert.py

📝 Walkthrough

Walkthrough

The PR fixes a serial number mismatch by assigning the generated certificate's serial number to the model instance (self.serial_number = str(cert.serial_number)) in _generate within django_x509/base/models.py. A test test_serial_number_db_matches_certificate was added to ensure the stored serial_number on the DB model matches the X.509 certificate's serial number when reloaded from the database.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely describes the main fix: ensuring the serial number stored in the database matches the certificate's serial number.
Description check	✅ Passed	The description covers all required sections from the template: checklist items (with appropriate marks), reference to issue #121, detailed explanation of the changes and root cause, and test information. Only documentation update was intentionally skipped.
Linked Issues check	✅ Passed	The pull request directly addresses issue #121 by fixing the serial number mismatch between the database and the certificate through synchronization after signing, and adds a test to verify the fix.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing the serial number mismatch issue: modifying _generate() to sync the serial number and adding a test to verify the fix. No out-of-scope changes are present.

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[coveralls]

coverage: 94.881% (+0.009%) from 94.872%
when pulling 39ee622 on devangpratap:master
into fc872b8 on openwisp:master.

[stktyagi]

Hey @devangpratap, can you reproduce and check the existence of this issue as this was fixed in #194 while migrating from PyOpenSSL to cryptography.