Unify the three keyed-mutex implementations into a shared package

[JAORMX]

Context

A duplication review of #5934 (server→client routing in the streamable proxy) found the repo now has three near-identical hand-rolled "per-key mutex" implementations of the same two-phase pattern (acquire a global mutex → look up/create a per-key sync.Mutex → release the global mutex → block on the per-key mutex):

• pkg/plugins/pluginsvc/service.go — pluginLock (never evicts; key cardinality bounded by installed plugins)
• pkg/skills/skillsvc/service.go — skillLock (never evicts; bounded)
• pkg/transport/proxy/streamable/keyed_mutex.go — keyedMutex (added in Route server-to-client MCP messages per session #5934; ref-counted / self-evicting because keys are arbitrary client-supplied resource URIs with unbounded cardinality, and it is deliberately held across a blocking upstream round-trip)

The keyedMutex variant intentionally diverges (eviction is required to avoid an unbounded-map leak — do not collapse it into the never-evicting variants), but the core acquire/release logic is duplicated three ways.

Proposed work

Extract a single generic keyed-mutex into an internal package (e.g. pkg/keyedmutex) supporting both modes:

• non-evicting (for the small, bounded pluginsvc/skillsvc key sets), and
• evicting / ref-counted (for unbounded, client-supplied keys like the streamable proxy's URIs).

Port all three call sites to it. Preserve each site's current semantics exactly (especially the streamable proxy's ref-counted eviction and its "held across a blocking call" usage — see the doc comment on keyedMutex).

Notes

• Not a behavior change; pure consolidation + tests for the shared package.
• Confirmed as a follow-up (not a blocker) by the Route server-to-client MCP messages per session #5934 review panel.

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