fix(spanner): include mutation_key in BeginTransaction RPC

[olavloite]

The BeginTransaction RPC should include a mutation_key when a transaction is only writing mutations.

[gemini-code-assist]

Code Review

This pull request implements mutation key selection to preserve blind write intent on multiplexed sessions by passing a selected mutation_key during explicit transaction initialization. It updates transaction-beginning methods across read_only_transaction.rs, read_write_transaction.rs, and result_set.rs to accept this optional key. Feedback on the changes suggests removing the newly introduced rand dependency in favor of a simpler, deterministic selection strategy (e.g., picking the first non-insert mutation). Additionally, the reviewer points out that matching on non-existent Operation::Send and Operation::Ack variants will cause compilation errors, and recommends removing the unused rand imports and dependencies.

[codecov]

Codecov Report

❌ Patch coverage is 98.71795% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.89%. Comparing base (265f1b4) to head (88dee17).

Files with missing lines	Patch %	Lines
src/spanner/src/mutation.rs	98.24%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5768   +/-   ##
=======================================
  Coverage   97.88%   97.89%           
=======================================
  Files         226      226           
  Lines       56447    56512   +65     
=======================================
+ Hits        55255    55322   +67     
+ Misses       1192     1190    -2     

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[alvarowolfx]

some ideas to avoid mut variables and also use existing methods to pick a random item on a iterator.

[alvarowolfx]

Maybe this can be more like this and avoid mut variables:

Suggested change

	let mut max_insert: Option<&crate::model::Mutation> = None;
	let mut max_rows = 0;
	for m in mutations {
	if let Some(Operation::Insert(write)) = &m.operation {
	let rows = write.values.len();
	if max_insert.is_none() || rows > max_rows {
	max_insert = Some(m);
	max_rows = rows;
	}
	}
	}
	let max_insert = mutations // Type: &[Mutation] (Slice)
	.iter() // 1. Converts to Iterator<Item = &Mutation>
	.filter_map(|m| match &m.operation { // 2. Filters and transforms
	Some(Operation::Insert(write)) => Some((m, write.values.len())),
	_ => None,
	}) // Now an Iterator<Item = (&Mutation, usize)>
	.max_by_key(|&(_, rows)| rows) // 3. Consumes iterator, returns Option<(&Mutation, usize)>
	.map(|(m, _)| m);

[alvarowolfx]

I think we can use the .choose method on iterators.

Suggested change

	let idx = select_random_index(non_inserts.len());
	return Some((*non_inserts[idx]).clone());
	use rand::seq::IteratorRandom;
	return non_inserts
	.iter()
	.choose(&mut rand::rng())
	.map(|&&m| m.clone());