[sonic-py-common] Add shared BGP ASN validation

[ashutosh-agrawal]

Why I did it

Several Python components need to apply the same validation when processing BGP ASNs. Keeping those checks in each component makes it easy for their behavior to drift.

Work item tracking

• Microsoft ADO (number only):

How I did it

Added a common validate_asn() helper to sonic-py-common.

The helper accepts decimal strings and integer values, rejects booleans and other representations, and checks the 32-bit BGP ASN range. Handling of missing values or component-specific sentinel values remains with the caller.

Added unit coverage for accepted values, range boundaries, invalid types, and invalid decimal representations.

Current consumers being updated to use this helper:

• bgpcfgd: Validate DEVICE_METADATA BGP ASN #29226 validates the DEVICE_METADATA BGP ASN before caching it.
• frr: Strictly validate isolate BGP ASN #29227 uses it as the sonic-cfggen filter for BGP isolation templates.
• frrcfgd: Validate BGP table key components #29230 uses it for BGP ASN fields processed by frrcfgd.

How to verify it

Run:

PYTHONPATH=src/sonic-py-common pytest -q --noconftest src/sonic-py-common/tests/test_bgp.py

Result: 22 passed.

Which release branch to backport (provide reason below if selected)

• 202305
• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605
• 202608

Tracking issue/work item for backport/cherry-pick request (GitHub issue or Microsoft ADO):
Failure type:

Tested branch

• master
• 202305
• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605
• 202608
• N/A

Test result

• master: 22 passed

Description for the changelog

Add a shared sonic-py-common helper for validating BGP ASNs.

Link to config_db schema for YANG module changes

N/A

A picture of a cute animal (not mandatory but encouraged)

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[mssonicbld]

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[mssonicbld]

/azp run Azure.sonic-buildimage

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[ashutosh-agrawal]

/azpw run

[mssonicbld]

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Retrying failed(or canceled) jobs...

[mssonicbld]

Retrying failed(or canceled) stages in build 1221391:

✅Stage Test:

• Job impacted-area-kvmtest-dualtor-aa-vpp by Elastictest: retried.