tests: Fix wrong expectations in bgp_srv6_unicast topotest

[cscarpitta]

After removing sid export on R1, the test checks both 10.0.0.1/32 and 10.0.0.3/32 on R2 with expect_sid="", expecting neither to carry a SRv6 SID. This is wrong: 10.0.0.3/32 is originated by R3 which still has sid export configured, so it should still be
seen on R2 with r3_unicast_sid.

This wrong expectation was not caught because check_route() did not verify the absence of a SID when expect_sid="".

Fix the check for 10.0.0.3/32 to expect r3_unicast_sid on R2, while keeping 10.0.0.1/32 checked for absence of SID.

[greptile-apps]

Greptile Summary

This PR fixes two related bugs in the bgp_srv6_unicast topotest that caused incorrect route SID verification.

• check_route() silent no-SID bypass (line 125–126): When expect_sid="", the original code relied on if expect_sid (which is falsy for an empty string) to drive the comparison, meaning the absence of a SRv6 SID was never actually asserted. The fix adds an explicit expect_sid == "" branch that returns an error if the route still carries a SID — closing a gap where incorrect SID presence would silently pass.
• Wrong expectation in test_bgp_srv6_sid_unexport() (lines 271–278): After R1's SID export is removed, only R1's own prefixes lose their SID. The 10.0.0.3/32 prefix originates from R3 (which retains sid export auto) and should still carry r3_unicast_sid on R2. The prior code incorrectly expected no SID for both 10.0.0.1/32 and 10.0.0.3/32. The fix separates the two checks: 10.0.0.1/32 is verified without a SID, while 10.0.0.3/32 is verified with r3_unicast_sid.

Confidence Score: 5/5

• This PR is safe to merge — it corrects two clearly identified test bugs with no production code impact.
• The changes are confined to a single test file, fix a genuine logic gap in check_route() (silent no-SID skip), and correct a provably wrong expectation for 10.0.0.3/32. Both fixes are consistent with the rest of the test's design and topology description. No ambiguity remains.
• No files require special attention.

Important Files Changed

Filename	Overview
tests/topotests/bgp_srv6_unicast/test_bgp_srv6_unicast.py	Two-part bug fix: (1) check_route() now correctly validates absence of a SRv6 SID when expect_sid="" was previously silently ignored; (2) test_bgp_srv6_sid_unexport() corrects the expectation for 10.0.0.3/32 to use r3_unicast_sid since R3 retains its SID export configuration.

Sequence Diagram

sequenceDiagram
    participant R1
    participant R2
    participant R3

    Note over R1: no sid export auto (unconfigured)
    Note over R3: sid export auto (still active)

    R1->>R2: Advertise 10.0.0.1/32 (no SRv6 SID)
    R2->>R2: Install 10.0.0.1/32 — expect_sid=""  ✓

    R3->>R1: Advertise 10.0.0.3/32 + r3_unicast_sid
    R1->>R2: Re-advertise 10.0.0.3/32 + r3_unicast_sid
    R2->>R2: Install 10.0.0.3/32 — expect_sid=r3_unicast_sid  ✓ (was wrongly "")

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