feat(sqlite): make thread_stack_size optional with safe defaults

[ZenasSong]

Changes thread_stack_size from usize to Option<usize> to address concerns about safety and platform compatibility.

Key improvements:

• Default to None, using Rust std's default stack size (typically 2MB)
• Only apply custom stack size when explicitly configured
• Safer for user callbacks with unpredictable stack requirements
• Platform-agnostic (handles 32-bit vs 64-bit differences automatically)
• Marked as an advanced option in documentation with appropriate warnings

This addresses the feedback from PR #3885 about hardcoded stack sizes being unsafe due to:

1. Unpredictable stack needs of user-supplied callbacks
2. Platform-specific requirements (32-bit vs 64-bit)
3. Need for conservative defaults

Related: #3885

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Is this a breaking change?

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If yes, this pull request will need to wait for the next major release (0.{x + 1}.0)

Behavior changes can be breaking if significant enough.
Consider Hyrum's Law:

With a sufficient number of users of an API,
it does not matter what you promise in the contract:
all observable behaviors of your system
will be depended on by somebody.

[abonander]

@ZenasSong please run cargo fmt.