[18.0][FIX] payroll_contract_advantages: bound validation, data integrity and precision

[CristianoMafraJunior]

What's broken today

• hr.contract.advantage._check_bound_limits() treats 0.0 as a real bound instead of
  "no limit on that side". Any hr.contract.advantage.template left with the default
  (unset) lower_bound/upper_bound therefore rejects every non-zero amount,
  making the advantage type unusable until an admin discovers they must also fill in
  bounds that have no required flag or hint suggesting so.
• Nothing prevents assigning the same advantage template twice on the same contract.
  payroll_contract_advantages/models/hr_payslip.py builds a {code: amount} dict from
  contract.advantages_ids, so a duplicate line silently overwrites one amount with the
  other with no warning, and the resulting payslip total is wrong without any error
  being raised.
• Nothing prevents two hr.contract.advantage.template records from sharing the same
  code. That code is the key salary rules use to read the value (advantages.<CODE>),
  so a duplicate code makes two different advantages collide in the same computation.
• hr.contract.advantage.contract_id is not required and has no ondelete, so
  deleting a contract leaves orphaned hr.contract.advantage rows with
  contract_id = NULL instead of being cleaned up.
• hr.contract.advantage.template allows saving a default_value outside its own
  lower_bound/upper_bound, and allows lower_bound > upper_bound — an
  already-inconsistent template that only surfaces the problem later, when someone tries
  to use it.
• amount (on the advantage line) and lower_bound / upper_bound / default_value
  (on the template) use plain Float with no digits precision, unlike the rest of the
  payroll engine which consistently uses the Payroll (2 decimals) precision for
  monetary-like amounts.

What this PR changes

• hr_contract_advantage.py
  • _check_bound_limits(): a bound of 0.0 is now treated as "not set", matching the
    common Odoo convention, instead of blocking every non-zero amount.
  • contract_id: now required=True, ondelete="cascade".
  • advantage_template_id: now required=True.
  • amount: now uses digits="Payroll".
  • New SQL constraint contract_template_unique: one template per contract.
• hr_contract_advantage_template.py
  • New SQL constraint code_unique.
  • New _check_bounds_consistency(): rejects lower_bound > upper_bound and a
    default_value outside the template's own bounds (same "0.0 = no limit"
    convention).
  • lower_bound / upper_bound / default_value: now use digits="Payroll".
• Tests: 5 new test cases covering every rule above, plus one pre-existing test adjusted
  (test_onchange_advantage_template_sets_default_amount) whose fixture value became
  inconsistent with the new bound-consistency check.

cc @nimarosa @pedrobaeza

[OCA-git-bot]

Hi @nimarosa,
some modules you are maintaining are being modified, check this out!