[xxwtiancai]

Link to Issue or Description of Change

Fixes #7706.

InMemoryArtifactService joined raw app_name/user_id/session_id/filename values into one flat storage key. The shared validator (artifact_util.validate_path_segment) deliberately allows embedded / in identifiers, so distinct (app_name, user_id) pairs — e.g. ("team/project", "alice") vs ("team", "project/alice") — addressed the same stored entry: the second scope could read the first's artifacts, allocate colliding version numbers, delete the first's artifacts, and see them in list_artifact_keys.

Key the store by a namespace tag plus percent-encoded segments instead:

• u/{enc(app)}/{enc(user)}/{enc(filename)} for user-namespace artifacts, s/{enc(app)}/{enc(user)}/{enc(session)}/{enc(filename)} for session artifacts, where enc percent-encodes with safe="" so an embedded / (%2F) can never shift a segment boundary.
• The leading u/s tag structurally separates the two namespaces (the existing reserved-segment guard on session ids stays as-is; the tag is defense-in-depth, not a replacement).
• list_artifact_keys matches these exact encoded prefixes and decodes filenames back, so its output format is unchanged; with no / in any encoded segment, a trailing-slash prefix now matches exactly one scope.
• The public artifacts field keeps its dict[str, list[_ArtifactEntry]] type; only the internal key encoding changes (existing tests already compute keys through _artifact_path, so nothing observable outside the service changes). The shared validator, canonical URIs, and every other backend are untouched — rejecting ambiguous identifiers would have been a behavior change for all backends and for users legitimately using / in app names.

Testing Plan

Unit tests and checks:

• Two new cross-scope regression cases in tests/unittests/artifacts/test_artifact_service.py, parametrized over both collision directions (("team/project", "alice") ↔ ("team", "project/alice")): the second scope sees an empty history (load/list/list_keys), its first save allocates its own version 0, the first scope's value and version history are untouched, and deleting through the second scope leaves the first intact.
• Revert control on unmodified main: both cases fail with the original flat key (the second scope reads the first's value); all pass with the fix.
• Full artifacts suite (all backends, including the shared conformance matrix): 1124 passed with the fix.
• Pre-commit on both changed files and mypy on the source file: clean.
• During analysis I also probed a potential second collision (session id user vs the user namespace): unreachable — artifact_util._validate_session_id_for_flat_storage already rejects such session ids at save time, so no test claims it; the namespace tag makes that invariant structural as well.

Manual end-to-end tests

Not needed beyond the suite: the service is entirely local, and the new cases exercise the public API exactly as the issue's repro does.

[adk-bot]

Thank you @xxwtiancai for your contribution! 🎉

Your changes have been successfully imported and merged via Copybara in commit 27cdf7f.

Closing this PR as the changes are now in the main branch.