Refactor ReaderModificationCommandBatch

[roji]

Note: this PR is based on #27573, review only the last commit.

This refactors ReaderModificationCommandBatch to simplify the adding of ModificationCommands, applying them directly to the batch's SQL and parameters rather than deferring that for later.

• Since SQL now gets add with every command, the SQL Server length check is now very cheap, and so I've removed the heuristic for doing it (we simply check every time). If we've gone over, we simply restart and re-add all the previous commands.
• I don't think that the scenario where we go over the SQL length is perf-critical, but if it is, we can optimize it by just rolling back the last command instead of replaying the entire batch.
• Note in our previous design, if a IsCommandTextValid occured in the middle of a pending bulk insert (eg. after 50 rows), that would cause that bulk insert to be terminated, resulting in multiple MERGE commands instead of one.

[AndriySvyryd]

I'll do the final review after #27573 is merged

[roji]

@AndriySvyryd merged #27573 and rebased this.

[AndriySvyryd]

No longer true

[roji]

But not entirely false either 😉

[ghost]

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[ghost]

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[ghost]

Apologies, while this PR appears ready to be merged, I've been configured to only merge when all checks have explicitly passed. The following integrations have not reported any progress on their checks and are blocking auto-merge:

1. Azure Pipelines

These integrations are possibly never going to report a check, and unblocking auto-merge likely requires a human being to update my configuration to exempt these integrations from requiring a passing check.

Give feedback on this

From the bot dev team

We've tried to tune the bot such that it posts a comment like this only when auto-merge is blocked for exceptional, non-intuitive reasons. When the bot's auto-merge capability is properly configured, auto-merge should operate as you would intuitively expect and you should not see any spurious comments.

Please reach out to us at fabricbotservices@microsoft.com to provide feedback if you believe you're seeing this comment appear spuriously. Please note that we usually are unable to update your bot configuration on your team's behalf, but we're happy to help you identify your bot admin.