test: ensure unit tests leave zero traces (DB + disk) (#951)

[ilayfalach]

Tests were leaking MongoDB projects and on-disk directories:

• A project stays in getProjectList() as long as any document carries its name, including the hidden config Cache document created on every Project() construction. Teardowns deleted only measurement docs, so the config doc kept every project alive.
• Project.init unconditionally creates ~/.hera/; nothing removed it.
• The session fixture persisted an ephemeral mkdtemp path into the durable DB config, so later Project() opens resurrected the tmp dir via makedirs.

Fix:

• Add purge primitives that delete all docs (incl. the config doc, via the collection layer) and remove on-disk dirs.
• Add a session-scoped autouse safety-net that purges every project created during the session plus leftover test-named projects/dirs. Only touches test artifacts; defaultProject and pre-existing data are never modified.
• Pass filesDirectory at construction so ~/.hera/ is never created, and purge the config doc on teardown so tmp dirs cannot be resurrected.
• dynamic_loading conftest now purges the DB, not just the disk dir.

Verified: 275 passed; zero new DB/dir/tmp artifacts; idempotent across runs.