fix(eval): generate Agent Engine-compliant eval session IDs

[SnowingFox]

Description

Fixes #6683

The default eval session ID generator in google.adk.evaluation.local_eval_service produced IDs of the form ___eval___session___<uuid>. The underscores violate the custom session ID constraints enforced by Vertex AI Agent Engine (only lowercase letters, digits and hyphens, with an alphanumeric first/last character). When VertexAiSessionService is configured, _get_or_create_eval_session() passes this generated ID straight to create_session(), Agent Engine rejects it with 400 INVALID_ARGUMENT, and the evaluation fails before inference starts.

This PR changes EVAL_SESSION_ID_PREFIX to eval-session-, so generated eval session IDs become eval-session-<uuid>, which satisfies the [a-z0-9-] constraint. The duplicate prefix constant in google.adk.cli.cli_eval (used by the API server to filter eval sessions out of session listings via startswith(...)) is updated in sync so eval sessions remain hidden from the listing.

Verification

Added a unit regression test tests/unittests/evaluation/test_eval_session_id.py asserting that _get_session_id() returns an ID matching ^[a-z0-9-]+$ with alphanumeric first/last characters. The test fails against the previous ___eval___session___<uuid> format and passes with the new eval-session-<uuid> format.

Notes

The issue also notes that VertexAiSessionService._SESSION_ID_PATTERN is more permissive than Agent Engine's real constraint. Tightening it would be a behavior change that could reject other currently-accepted session IDs (several existing tests use underscore-containing session IDs), so this PR keeps the fix minimal and only corrects the generated eval session IDs.

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