test(policy): cover new IsDeprecatedSupport Yellow branch

[Kiran01bm]

What and Why ?

Follow-up to the deletes-only fix that landed alongside #66. PR #66 added
a new IsDeprecatedSupport branch to getYellowMessage (Lambda-style
"Standard Support" / "Deprecated Support" semantics) plus the
versionSubject and isLambda helpers, but no tests were added at the
time. The deletes-only fix dropped policy coverage from 98.8% → 96.0%
because it removed the GetRecommendation test surface that #66 had
already deleted; this PR recovers that coverage by exercising the new
branch instead.

Tests added

Test	Branch covered
TestGetMessage_Yellow_DeprecatedSupport_WithEOL	Lambda runtime in deprecated support with an end date — exercises the IsDeprecatedSupport+EOLDate path and versionSubject's "Runtime" label (via isLambda)
TestGetMessage_Yellow_DeprecatedSupport_NoEOL	IsDeprecatedSupport without an end date — non-Lambda resource, so versionSubject returns "Version"; asserts the until-phrase is omitted

Verification

All commands run with GOTOOLCHAIN=go1.24.2:

• go test ./pkg/policy/... -cover → 98.7% (up from 96.0%)
• go test ./... → all packages green
• golangci-lint run ./... → 0 findings