truncate should always be treated as warning in cast decimal as decimal

[windtalker]

What problem does this PR solve?

Issue Number: close #7348

Problem Summary:
Accoring to TiDB's implementation, truncate in cast decimal as decimal should be always treated as a warning instead of error.
https://github.com/pingcap/tidb/blob/96fa4692b09b43ae587ea449e5891a9d9fd6e5c8/types/datum.go#L1512-L1519

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• guo-shaoge
• yibin87

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[yibin87]

LGTM

[windtalker]

/merge

[ti-chi-bot]

@windtalker: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: a28986e

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #7357.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #7358.

[JaySon-Huang]

/cherry-pick release-6.5-20230425-v6.5.2

[ti-chi-bot]

@JaySon-Huang: new pull request created to branch release-6.5-20230425-v6.5.2: #7378.

In response to this:

/cherry-pick release-6.5-20230425-v6.5.2

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.