Fix ClearFlag CLI flag mapping

[Copilot]

High Level Overview of Change

xrpl account clear-flag was wiring the requested flag into set_flag, so the CLI did the opposite of the user intent. This corrects the transaction construction and adds a regression check for the emitted AccountSet fields.

Context of Change

The bug was in the ClearFlag CLI path: a clear request could serialize as a flag set, including asfDisableMaster, which can lock an account out of master-key authorization.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Tests (You added tests for code that already exists, or your new feature included in this PR)

Before / After

• SetFlag path stays unchanged.
• ClearFlag now populates clear_flag and leaves set_flag unset.

// before
AccountSet::new(..., None, ..., None, Some(flag_enum), ...);

// after
AccountSet::new(..., Some(flag_enum), ..., None, None, ...);

• Added a CLI regression test that decodes the signed transaction blob and verifies:
  • ClearFlag is present
  • SetFlag is absent

Test Plan

• cargo test --features "std cli integration" --test cli_integration test_account_clear_flag -- --nocapture
• cargo test --features "std cli integration" --test cli_integration test_account_set_flag -- --nocapture