ddl: make args/DecodeArgs un-exported

[D3Hunter]

What problem does this PR solve?

Issue Number: ref #54436

Problem Summary:

What changed and how does it work?

• make args in job/subjob and DecodeArgs unexported, now caller should use GetXXXArgs function instead
• remove change fields inside job.decodeArgs
• remove tests related to MockModifyJobArg failpoint, it only test how we handle corrupted args, and the job is cancelled in this case.

Check List

Tests

• [x] Unit test, use existing cases to cover it, we mainly rename fields/methods
• [ ] Integration test
• [ ] Manual test (add detailed scripts or steps below)
• [ ] No need to test

  • [ ] I checked and no code files have been changed.

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

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None