evalengine: fix numeric coercibility

[vmg]

Description

@systay discovered this issue during fuzzing. We need special handling for the coercibility of numeric types in the evalengine, otherwise some case expressions will not be collated as text as you'd expect.

We've fixed the issue by replacing the generic defaultCoercionCollation helper with an enhanced version that takes into account the type of the value being collated. This also fixes mismatched collations for some datetime and JSON values.

Here's an example of an expression that had the right return type, but the wrong collation before this change:

			expression: `case column0 when 1 then column1 else column2 end`,
			values:     []sqltypes.Value{sqltypes.NewInt64(42), sqltypes.NewVarChar("sole"), sqltypes.NewInt64(0)},
			result:     `VARCHAR("0")`,
			collation:  collations.CollationUtf8mb4ID,

cc @dbussink

Related Issue(s)

Fixes #14480

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[vmg]

This was not being initialized as Unknown, so it was propagating as NULL.

[vmg]

Same here. This was not being initialized as Unknown.

[dbussink]

Thanks for the fuzzer to find that we're not doing this right before and using the wrong coercions!