connection_manager: replace usize counter with Semaphore RAII permit

[amankrx]

Description

Production worker pods were OOMKilled (exit 137) after the ConnectionManager's available_connections: usize counter underflowed to ~u64::MAX while waiting_connections climbed unbounded. The manual decrement-on-issue / increment-on-Dropped accounting balances on paper, but a leak path was occasionally missing a Dropped delivery during tonic transport errors and task aborts.
Switch to Arc with OwnedSemaphorePermit on the Connection. RAII makes leakage structurally impossible: every Drop path (panic, abort, dropped oneshot receiver, transport error) releases the permit exactly once.

Fixes # (issue)

Type of change

Please delete options that aren't relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Added tests and tested it in an actual running environment.

Checklist

• Updated documentation if needed
• Tests added/amended
• bazel test //... passes locally
• PR is contained in a single commit, using git amend see some docs

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This change is 

[palfrey]

Minor naming item, otherwise good

[MarcusSorealheis]

I like the use of a permit here in place of a counter.

[MarcusSorealheis]

Nice