31800: validate Incorporation Application share structure fields

[loneil]

Issue #: /bcgov/entity#31800

Description of changes:
WIP
Needs bcgov/business-schemas#202 merged so unit tests can be accomodated

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[Copilot]

Pull request overview

This PR tightens validation rules for share structures, enforcing consistent naming for share classes and series and adding comprehensive unit tests around these rules.

Changes:

• Introduced EXCLUDED_WORDS_FOR_CLASS and EXCLUDED_WORDS_FOR_SERIES constants and extended validate_shares / validate_series to enforce a " Shares" suffix and to reject reserved words in class and series names.
• Ensured series-level validations (max shares, rights/restrictions, duplicate names) now also include suffix and reserved-word checks while maintaining existing behavior.
• Added extensive unit tests in test_common_validations.py for share-class and series names, as well as for end-to-end validate_share_structure behavior over various valid and invalid share structures.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
legal-api/src/legal_api/services/filings/validations/common_validations.py	Adds reserved-word and suffix validations for share class and series names, and exposes exclusion word lists as constants used across filings.
legal-api/tests/unit/services/filings/validations/test_common_validations.py	Adds parameterized tests covering class/series name suffix rules, reserved words, whitespace/emptiness, duplication, and full share-structure validation behavior.

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