fix: DynamoDB placeholder generation no longer overwrites caller values

[TrueFurina]

Fixes #4831

Problem: ConditionExpressionBuilder restarts placeholder numbering at #n0/:v0 on every request, and the generated placeholders are merged over the caller's with dict.update — so a caller-supplied #n0/:v0 gets silently overwritten and the expression resolves to the wrong value.

Fix: after reset(), start placeholder numbering after the highest #n / :v index the caller already provided, so generated placeholders never collide with the caller's ExpressionAttributeNames/ExpressionAttributeValues.

[TrueFurina]

Nudging this politely. The behavioral delta is narrow: dynamodb.types placeholder generation could overwrite entries in the caller's ExpressionAttributeValues/Names dicts when the same placeholder was regenerated, so mutating callers (reusing a dict across update_item calls) saw values silently replaced. After the fix the generator copies before merging. No checks are configured on this fork branch, so pending likely just reflects that. If boto3's direction is that callers must not reuse these dicts, I'm fine closing this in favor of a docs note instead — guidance appreciated.