## What's Changed

## Fix Critical Deadlocks and Race Conditions in Snowflake Record Reader

This PR addresses multiple critical concurrency issues in the Snowflake
driver's `recordReader` that could cause complete application hangs
under normal racing conditions.

### Issues Fixed

*1. Critical Deadlock: `Release()` Blocking Forever*

*Problem*: When `Release()` was called while producer goroutines were
blocked on channel sends, a permanent deadlock occurred:

* `Release()` cancels context and attempts to drain channels
* Producer goroutines blocked on `ch <- rec` cannot see the cancellation
* Channels never close because producers never exit
* `Release()` blocks forever on `for rec := range ch`

*Fix:* Added a `done` channel that signals when all producer goroutines
have completed. `Release()` now waits for this signal before attempting
to drain channels.

*2. Severe Deadlock: Non-Context-Aware Channel Sends*

*Problem:* Channel send operations at lines 694 and 732 checked context
before the send but not during:

```go
for rr.Next() && ctx.Err() == nil {  // Context checked here
    // ... 
    ch <- rec  // But send blocks here without checking context
}
```

*Fix:* Wrapped all channel sends in `select` statements with context
awareness:

```go
select {
case chs[0] <- rec:
    // Successfully sent
case <-ctx.Done():
    rec.Release()
    return ctx.Err()
}
```

*3. Critical Race Condition: Nil Channel Reads*

*Problem:* Channels were created asynchronously in goroutines after
`newRecordReader` returned. If `Next()` was called quickly after
creation, it could read from uninitialized (nil) channels, causing
infinite blocking.

*Fix:* Initialize all channels upfront before starting any goroutines:

```go
chs := make([]chan arrow.RecordBatch, len(batches))
for i := range chs {
    chs[i] = make(chan arrow.RecordBatch, bufferSize)
}
```

*4. Goroutine Leaks on Initialization Errors*

*Problem:* Error paths only cleaned up the first channel, potentially
leaking goroutines if initialization failed after starting concurrent
operations.

*Fix:* Moved all error-prone initialization (GetStream, NewReader)
before goroutine creation, and added proper cleanup on errors.

----------------------

#### Changes

* Added `done` channel to `reader` struct to signal goroutine completion
* Initialize all channels upfront to eliminate race conditions
* Use context-aware sends with `select` statements for all channel
operations
* Update `Release()` to wait on `done` channel before draining
* Reorganize initialization to handle errors before starting goroutines
* Signal completion by closing `done` channel after all producers finish

#### Reproduction Scenarios Prevented

*Deadlock:*

1. bufferSize = 1, producer generates 2 records quickly
2. Channel becomes full after first record
3. Producer blocks on send
4. Consumer calls Release() before Next()
5. Without fix: permanent deadlock
6. With fix: producer responds to cancellation, Release() completes

*Race Condition:*

1. Query returns 3 batches
2. First batch processes quickly
3. Next() advances to second channel
4. Without fix: reads from nil channel, blocks forever
5. With fix: channel already initialized, works correctly

Backport of apache/arrow-adbc#3870.