fix(analytics): Infinite unbound loop in analytics processor

[khvn26]

Closes #55.

In this PR, we:

1. Fix a busy analytics processor loop by using recv_timeout.
2. Make sure the processor is flushed on shutdown.

Measured on an idle processor with no features tracked, over one second of wall clock:

	CPU consumed
before	0.95s (~95% of a core)
after	0.00s

The loop now blocks for whatever remains of the flush window, so it wakes only when a feature is tracked or a flush is due. Flush cadence is unchanged — a Timeout just falls through to the existing flush check. The write lock is taken only when there is data to record or a flush to perform.

Disconnected previously broke out of the loop immediately. With the default 10s timer, dropping a client could silently lose up to 10s of analytics. Shutdown now flushes unconditionally before exiting, on a best-effort guarantee (we don't want to add a handle as part of this PR — tracking in #58).

The remaining wait uses timer.saturating_sub(elapsed). Without a guard, a flush that overran its own window (a slow HTTP POST, say) makes timer - elapsed underflow: a panic in debug, and in release a ~584M year timeout, which would wedge the analytics thread permanently with no further flushes. That was reachable on the original code path too.

Added two tests to cover new code.

[matthewelwell]

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📝 Walkthrough

Walkthrough

The analytics worker now waits for incoming events or the next flush deadline instead of polling. It updates analytics data when events arrive, flushes on timer expiry or channel disconnect, clears flushed data, and exits after shutdown. Tests cover flushing pending events when the processor is dropped and handling an unreachable endpoint without panicking.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🟡 Moderate · up to 4ceea

The shutdown path may return before pending analytics are flushed, so applications that exit immediately can lose recent analytics data; merge should wait for the flush to complete or obtain explicit owner acceptance of this bounded risk.

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• src/flagsmith/analytics.rs

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