chore(release): move the package validation baseline to 2.6.0

[Harol-Reina]

Step two of the release ritual — the same shape #265 took after 2.5.3. With 2.6.0 published, ApiCompat
compares against it instead of against 2.5.3, which makes all fourteen CompatibilitySuppressions
entries obsolete.

What goes

file	entries
src/Verbara.Sdk/CompatibilitySuppressions.xml	2
src/Verbara.Sdk.Ari/CompatibilitySuppressions.xml	6
src/Verbara.Sdk.VoiceAi.AudioSocket/CompatibilitySuppressions.xml	6

They existed so the AudioSocket frame-type corrections (#302) and the CreateExternalMediaAsync
signature change (#305) could pack against a 2.5.3 baseline that still held the old surface. Against
2.6.0 there is no diff to suppress.

They move with the baseline rather than later on purpose. Left in place, they would silently mask
the next break of the same shape — an enum value changed, a member removed — and nothing would
report it.

Verified with a negative control, not with a green pack

dotnet pack validation is incremental, so a gate that never ran reports success. Both directions
were run:

baseline 2.6.0, no suppressions   ->  exit 0, 29 packages, 0 errors, 0 warnings
baseline 2.5.3, no suppressions   ->  exit 1, 6 ApiCompat errors (2x CP0002, 4x CP0011)
                                      "CP0011: Value of field 'Uuid' in enum 'AudioFrameType'
                                       changed from '0' to '1'" + five more

The six errors in the failing direction are exactly the six the Ari file used to carry. The gate is
live, and the suppressions really were what was holding it.

Release state

2.6.0 is tagged (v2.6.0 → e1a731fe), publish.yml succeeded, and 29 of 29 packages are indexed
on nuget.org — checked package by package against the flat container rather than trusting the
workflow's own canary poll, which covers one package.

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