[19.0][IMP] base_tier_validation_formula: code widget on python expressions

[bosd]

Summary

• Apply widget="code" with options="{'mode': 'python'}" to reviewer_expression, so the Python Expression reviewer field gets the same syntax-highlighted editor as python_code.
• Move python_code into its own full-width group (col="4" / colspan="4") so the code editor is no longer squeezed to half-width inside the default 2-column Apply On group.
• Add placeholder hints on both fields listing the available locals (rec, expected return type) for the first time a user clears the default.

Test plan

• Open a tier.definition record, set Review Type = Python Expression: the Review Expression field renders with the code widget (syntax highlighting, monospace) and takes full sheet width.
• Set Definition Type = Formula or Domain & Formula: the Tier Definition Expression field is shown in a full-width code editor.
• Clear each expression: the placeholder lists the available locals.
• Switch Review/Definition Type back to a non-expression value: the fields hide as before.

Before:

After:

[bosd]

Heads up @santostelmo: I force-pushed a tiny housekeeping change — dropped the manual __manifest__.py version bump (per @LoisRForgeFlow's request across all open PRs; ocabot bumps on merge). No functional change to the code you approved. CI is green.