* fix(plugins): add thread-safe lazy-singleton helpers, fix honcho TOCTOU (#24759)
get_honcho_client() and fal's _load_fal_client() used unlocked
check-then-init: racing threads both ran the expensive build and the
loser's client (open connection) leaked.
Rather than one-off locks, add plugins/plugin_utils.py with two
reusable primitives every plugin author can drop in:
- lazy_singleton: decorator for zero-arg accessors
- SingletonSlot: manual slot for config-keyed accessors (first wins)
Both use double-checked locking; factory runs at most once; failed
builds aren't cached. honcho is the reference consumer; fal's sibling
TOCTOU gets a matching double-checked lock. Plugin dev guide documents
the pattern so future plugins don't reintroduce the race.
Closes#24759
* test(honcho): update reset test for SingletonSlot internals
test_reset_clears_singleton poked the removed _honcho_client module
global directly. Assert through the slot's public peek() surface
instead, matching the #24759 refactor.
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