Added a new testcase for macsec_gearbox

[shreyansh-nexthop]

Why I did it

• Add a test case for mixed MACsec support across multiple gearbox PHYs, where some PHYs have macsec_supported=true and others have macsec_supported=false. This validates the scenario where a platform owner enables MACsec support only for specific gearbox PHYs. Got this as part of review comment for PR#3926.
• Made the helper functions more robust by checking if the macsec is configured for the right interface or not.

How I did it

Added test_macsec_mixed_phy_support test case that:

• Dynamically creates a second PHY (PHY 2) by reassigning an existing interface
• Configures PHY 1 with macsec_supported=true (uses PHY backend → GB_ASIC_DB)
• Configures PHY 2 with macsec_supported=false (uses NPU backend → ASIC_DB)
• Enables MACsec on ports from both PHYs
• Verifies MACsec keys are created in the correct database based on the PHY's macsec_supported setting

Added/Modified helper functions in gearbox.py:

• reassign_interface_to_phy(): Reassigns an existing interface to a new PHY (creates the PHY if needed)
• get_gearbox_port_by_phy(): Gets a port connected to a specific PHY
• verify_macsec_for_port_in_asic_db() and verify_macsec_for_port_in_gb_asic_db: Modified the old helper functionsThis is more robust as it verifies if the macsec is configured for the right interface or not.

How to verify it

It can be verified by running the testcase TestMacsecGearbox::test_macsec_mixed_phy_support

Which release branch to backport

• 202505

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: shreyansh-nexthop / name: shreyansh-nexthop (7a965a0)

[mssonicbld]

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[shreyansh-nexthop]

Hi, @judyjoseph, can you please help with the review? This is based on the review comment given on PR#3926.

I went ahead with the dynamically generating phy approach, as adding a second PHY to brcm_gearbox_vs/gearbox_config.json in sonic-buildimage won't work. It leads to failure of gearbox testcases, since multiple phys is not supported.

File: src/sonic-sairedis/vslib/VirtualSwitchSaiInterface.cpp

if (m_switchStateMap.size() == 1) {
    switch_id = m_switchStateMap.begin()->first;
} else {
    SWSS_LOG_THROW("multiple switches not supported, FIXME");
}

[rajshekhar-nexthop]

Hi @prsunny , Please help to get this PR reviewed. Thanks

[prsunny]

@judyjoseph , please review and signoff. Lgtm

[prsunny]

@shreyansh-nexthop , could you rebase?

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