polish: refactor functions exceeding 50-line soft limit in codegen modules

[krystophny]

Several functions in the new codegen modules exceed the 50-line soft limit (though all are under the 100-line hard limit):

codegen_character_normalization.f90:

• extract_character_length: 54 lines

codegen_declaration_grouping.f90:

• can_group_declarations: 53 lines
• can_group_declarations_with_params: 58 lines

codegen_grouped_body.f90:

• process_grouped_declarations: 84 lines
• process_grouped_parameters: 59 lines

Recommendation

Consider further refactoring to split these into smaller helper functions when convenient. This is a style polish item, not a blocker - all functions are well under the 100-line hard limit and tests pass.

Context

These modules were created in PR #1947 to split the monolithic codegen_declarations_programs.f90 file and comply with module size limits. The hard limit compliance was enforced in commit 71ede28.