test: fill coverage gaps in `discovery/`, `code_utils/`, `models/`, `github/`, `either.py`

[KRRT7]

Parent: #2079

Phase 3 — Medium-high coverage to 100% (69–86%)

Module	Stmts	Current coverage
discovery/	1,200	69%
code_utils/	2,634	71%
github/	34	71%
either.py	27	78%
models/	863	86%

discovery/

• functions_to_optimize.py (539 stmts, 64%)
• discover_unit_tests.py (619 stmts, 78%)
• pytest_new_process_discovery.py (42 stmts, 0%)

code_utils/

Key files needing attention:

• code_utils.py (308 stmts, 56%)
• env_utils.py (119 stmts, 49%)
• shell_utils.py (159 stmts, 60%)
• config_parser.py (149 stmts, 62%)
• tabulate.py (524 stmts, 63%)
• git_utils.py (125 stmts, 65%)
• git_worktree_utils.py (121 stmts, 19%)

models/

• models.py (636 stmts, 82%) — cover remaining model methods, serialization edge cases
• test_type.py (11 stmts, 0%)

github/, either.py

Small gap-fills to reach 100%.

Target

100% coverage for these modules (~4,758 statements).