Fix Phoenix socket reconnection

[atdrendel]

• Restore Phoenix reconnection backoff
• Remove race condition resulting in no reconnection attempts when a connection attempt fails
• Do not wait for channel leave to succeed before trying to rejoin the channel

[Copilot]

Pull request overview

This PR focuses on making Phoenix socket reconnection behavior more reliable by restoring reconnection backoff, avoiding a reconnection “stall” when an onClose happens during a failed connection attempt, and allowing channels to proceed with rejoin logic without blocking on leave completion.

Changes:

• Restores fractional reconnect backoff timing during repeated connection attempts.
• Adjusts socket close handling to avoid cancelling the reconnect task during .connecting failures.
• Updates channel join-timeout flow to fire the leave asynchronously and immediately schedule a rejoin; adds regression tests for reconnection scenarios.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
Tests/PhoenixTests/PhoenixSocketTests.swift	Adds regression tests covering backoff application and reconnection continuation across onClose + open failures.
Sources/Phoenix/PhoenixSocket.swift	Fixes backoff sleep precision and adjusts close/reconnect task cancellation behavior.
Sources/Phoenix/PhoenixChannel.swift	Renames join-timeout error type and makes leave-after-timeout fire asynchronously to avoid blocking rejoin scheduling.

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