fix(conn): conn to have state machine

[ndyakov]

• Add connection state tracking instead of multiple flags
• Resolve race conditions on handoff
• Improve pool performance

This PR fixes issues related to us having multiple different atomic flags in the pool connections for handoff, usability, etc. Since I tried to slightly changed the get and set operations of the pool, this correctness (which negatively impacted the performance in the first place) does not have real world drawbacks and improves developer experience.

Also fixes:

• fixes Misleading error log in pool dialConn if DialerRetryTimeout reached #3593
• fixes Connection pool issues after upgrading to v9.16.0 #3582

[Copilot]

Pull Request Overview

This PR refactors connection management to use a state machine architecture, replacing the previous boolean flags (usable, Inited, used) with a formal state machine that provides better concurrency control and clearer state transitions.

Key Changes:

• Introduced a new ConnStateMachine with explicit states: CREATED → INITIALIZING → IDLE ⇄ IN_USE, with UNUSABLE for handoff/reauth operations
• Replaced atomic boolean flags with atomic state transitions using CAS operations and FIFO waiting queues
• Updated connection initialization logic to handle concurrent initialization attempts properly

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
internal/pool/conn_state.go	New file implementing the connection state machine with FIFO waiting queue
internal/pool/conn_state_test.go	Comprehensive tests for state machine transitions and concurrency
internal/pool/conn.go	Refactored connection to use state machine instead of boolean flags
internal/pool/pool.go	Updated pool operations to use state machine transitions
internal/pool/pool_single.go	Added comments explaining SingleConnPool's non-thread-safe nature
redis.go	Refactored initConn to handle state transitions and concurrent initialization
maintnotifications/pool_hook.go	Updated to check handoff state before usability
maintnotifications/pool_hook_test.go	Updated tests to use state machine API
maintnotifications/handoff_worker.go	Added call to clear handoff state before closing
maintnotifications/errors.go	Added new error type and documentation comments
internal/auth/streaming/pool_hook.go	Simplified reauth logic using state machine's AwaitAndTransition
internal/auth/streaming/pool_hook_state_test.go	New tests verifying reauth respects state machine

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[Copilot]

Pull Request Overview

Copilot reviewed 15 out of 15 changed files in this pull request and generated 6 comments.

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[Copilot]

Pull Request Overview

Copilot reviewed 34 out of 37 changed files in this pull request and generated 4 comments.

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[htemelski-redis]

Overall looks good.
There aside from the other comments, can we preallocate all errors.New(...) too?