docs: Document transaction failure exceptions

[loopassembly]

Summary

• Add guidance for exceptions thrown when transactions fail.
• Document that database-rejected queries inside a transaction surface as DatabaseQueryException.
• Show how to compare DatabaseQueryException.code with PgErrorCode.serializationFailure and PgErrorCode.deadlockDetected.
• Fix the transactions isolation table markdown formatting and a PostgreSQL typo in the touched page.

Verification

• npx --yes --cache ./.npm-cache markdownlint-cli docs/06-concepts/06-database/08-transactions.md
• git diff --check

I did not run a docs build because serverpod_docs/AGENTS.md says not to run build commands locally.

Closes serverpod/serverpod#3920

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds guidance on handling exceptions thrown during transactions and clarifies how to detect/retry on common PostgreSQL concurrency errors, while also tightening up the transaction isolation table formatting/spelling.

Changes:

• Documented how Serverpod rolls back/rethrows on transaction callback exceptions and how database rejections surface as DatabaseQueryException.
• Added a Dart example for checking DatabaseQueryException.code against PgErrorCode (e.g., serialization failure, deadlock).
• Updated the transaction isolation table header formatting and corrected “PostgresSQL” → “PostgreSQL”.

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