fix: make python -m tabcmd work reliably (#375)

[jacalata]

Summary

• Fixes Add tabcmd/__main__.py so python -m tabcmd works reliably for installed users #375
• Restores if __name__ == "__main__": guard (removed guard would call sys.exit() on any import tabcmd.__main__)
• Adds sys.exit(1) in the import-failure handler so the process exits non-zero instead of crashing on main() being undefined
• Broadens except ImportError to except Exception — broken installs can also raise SyntaxError, OSError (missing .mo files), AttributeError
• Makes e.__context__ conditional (was always printing None for top-level import failures)
• Improves error message to suggest --force-reinstall (tabcmd is already on sys.path when this fires)
• Routes all diagnostic prints in the frozen-bundle path to stderr
• Adds tests/test_main.py with subprocess-based tests; import-error test uses sys.modules injection instead of fragile CWD/path ordering; success tests pass PYTHONPATH explicitly

Test plan

• python -m pytest tests/test_main.py -v — 4 tests pass
• python -m pytest tests/ --ignore=tests/e2e — full suite passes
• python -m tabcmd exits 0
• python -m tabcmd --help exits 0 and prints usage
• Broken install path: exits 1 with "Exception thrown importing tabcmd" on stderr

🤖 Generated with Claude Code

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
tabcmd
__main__.py	12	12	12	0%
tabcmd.py	15	15	15	0%
version.py	6	2	2	67%
tabcmd/commands
commands.py	10	10	10	0%
constants.py	77	18	18	77%
server.py	135	18	18	87%
tabcmd/commands/auth
session.py	381	46	46	88%
tabcmd/commands/datasources_and_workbooks
datasources_and_workbooks_command.py	157	18	18	89%
datasources_workbooks_views_url_parser.py	142	5	5	96%
delete_command.py	60	16	16	73%
export_command.py	120	25	25	79%
get_url_command.py	127	47	47	63%
publish_command.py	123	28	28	77%
runschedule_command.py	21	7	7	67%
tabcmd/commands/extracts
create_extracts_command.py	42	8	8	81%
decrypt_extracts_command.py	27	2	2	93%
delete_extracts_command.py	37	6	6	84%
encrypt_extracts_command.py	27	2	2	93%
extracts.py	20	2	2	90%
reencrypt_extracts_command.py	27	2	2	93%
refresh_extracts_command.py	48	10	10	79%
tabcmd/commands/group
create_group_command.py	29	5	5	83%
delete_group_command.py	27	2	2	93%
tabcmd/commands/project
create_project_command.py	46	8	8	83%
delete_project_command.py	35	4	4	89%
publish_samples_command.py	30	4	4	87%
tabcmd/commands/site
create_site_command.py	34	5	5	85%
delete_site_command.py	27	2	2	93%
edit_site_command.py	38	2	2	95%
list_command.py	77	12	12	84%
list_sites_command.py	29	2	2	93%
tabcmd/commands/user
add_users_command.py	29	5	5	83%
create_site_users.py	58	11	11	81%
create_users_command.py	60	10	10	83%
delete_site_users_command.py	43	5	5	88%
user_data.py	221	31	31	86%
tabcmd/execution
_version.py	2	2	2	0%
global_options.py	125	8	8	94%
localize.py	66	11	11	83%
logger_config.py	51	6	6	88%
tabcmd_controller.py	42	7	7	83%
TOTAL	2837	441	84%