Add agent persona methods to the client

[tomchop]

Adds search_agent_personas, get_agent_persona, new_agent_persona and patch_agent_persona, covering Yeti's /api/v2/agentpersonas endpoints.

Why

yeti-agents reads and seeds personas over this API. Without these it builds the URLs itself:

f"{client._url_root}/api/v2/agentpersonas/search"

_url_root is private, and nothing obliges this class to keep it. Every other Yeti call in yeti-agents already goes through a typed method here — semantic_search was added for exactly this reason. Personas are ordinary Yeti objects with an ordinary CRUD API, so they belong alongside the other object types rather than being reconstructed by each caller, along with the response envelope ({"personas": [...], "total": n}) that callers currently unpack by hand.

Two deliberate departures from the neighbouring methods

No required arguments on search. search_entities and friends raise if you pass nothing, because an unfiltered dump of entities is rarely what anyone meant. For personas the opposite is true — listing all of them is the common case, and Yeti's PersonaSearchRequest already defaults name to "", meaning match-all.

enabled is three-valued. None means "either" and is omitted from the payload; False has to be sent, since it is a real filter and not an absent one. Guarding with if enabled: would silently turn a search for disabled personas into a search for all of them, so it is guarded with is not None and there is a test pinning it.

No delete method

do_request dispatches GET, POST and PATCH and raises ValueError on anything else, so DELETE /agentpersonas/{id} is not reachable from this client. Adding it means teaching do_request a new verb — out of scope here, and no caller needs it yet.

Testing

6 new tests, 37 total passing, matching the existing mock-the-Session style: each asserts both the parsed return value and the exact URL and payload sent.

Ran the three CI jobs locally against a fresh poetry install --no-root:

=== pyrefly ===   INFO 0 errors (1 warning not shown)
=== unittest ===  Ran 37 tests in 0.031s  OK
=== ruff ===      6 files already formatted

No dependency changes, so poetry.lock is untouched.

Follow-up

yeti-platform/yeti-agents#12 is held pending this: once released, it drops the private-attribute access and the hand-built endpoint constant in favour of these methods.