fix: prompt for password when --username given without credentials (#301)

[jacalata]

Summary

• When --username is provided but no password or token, and there is no active session, tabcmd now prompts for a password (or exits cleanly if --no-prompt is set)
• Does not sign out an existing active session when only --username is passed — the session reuse path is preserved

Closes #301

Test plan

• test_create_session_username_only_prompts_for_password — getpass called when no session active
• test_create_session_username_only_reuses_existing_session — no prompt when session exists
• test_create_session_username_only_no_prompt_exits — exits cleanly with --no-prompt

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
tabcmd
__main__.py	10	10	10	0%
tabcmd.py	14	14	14	0%
version.py	6	3	3	50%
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	372	43	43	88%
tabcmd/commands/datasources_and_workbooks
datasources_and_workbooks_command.py	156	18	18	88%
datasources_workbooks_views_url_parser.py	140	10	10	93%
delete_command.py	60	16	16	73%
export_command.py	119	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
create_site_users.py	57	10	10	82%
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	42	6	6	86%
tabcmd_controller.py	42	7	7	83%
TOTAL	2805	435	84%