tech-debt(constants): complete error_constants adoption — unused ERR_FAILED_TO/ERR_NOT_FOUND and ~20 remaining hardcoded sites

[mrveiss]

Context

PR #3539 (closes #3530) created constants/error_constants.py and wired it into 6 files. A post-merge code review identified incomplete adoption.

Issues

Dead constants — never adopted

• ERR_FAILED_TO = "Failed to {operation}" — defined and exported but zero call sites updated. The 40+ "Failed to" sites that motivated this constant were not replaced.
• ERR_NOT_FOUND = "{resource} not found" — same: defined but never used with .format() at call sites.

Remaining hardcoded sites (~20)

File	Detail string	Constant to use
api/templates.py:306,334,426	"Template not found"	ERR_TEMPLATE_NOT_FOUND
api/scheduler.py:440	"Template not found"	ERR_TEMPLATE_NOT_FOUND
api/scheduler.py:149	"Workflow not found"	ERR_WORKFLOW_NOT_FOUND
services/workflow_automation/routes.py:127,181,344,358,487	"Workflow not found"	ERR_WORKFLOW_NOT_FOUND
api/advanced_control.py:130,361	"Session not found"	ERR_SESSION_NOT_FOUND
api/agent_terminal.py:495,522	"Session not found"	ERR_SESSION_NOT_FOUND
api/research_browser.py:126,176,205,302,322	"Session not found"	ERR_SESSION_NOT_FOUND

Minor

• Comment on ERR_NOT_FOUND at error_constants.py:6 uses unnecessary f"..." wrapper: f"{ERR_NOT_FOUND.format(resource='Workflow')}" → should be ERR_NOT_FOUND.format(resource='Workflow').

Work required

1. Replace all ~20 remaining hardcoded detail= strings listed above with the appropriate constant
2. Either adopt ERR_FAILED_TO at the top 10+ "Failed to" call sites, or remove it from the constants file and __init__.py if not viable
3. Either adopt ERR_NOT_FOUND via .format() at generic 404 sites, or remove it
4. Fix the comment typo on error_constants.py:6

Notes

• api_endpoint_migrations_test.py inspect.getsource() assertions must be updated for each newly migrated file (follow the pattern established in PR refactor(constants): create error_constants.py — centralize repeated error strings (#3530) #3539)
• ERR_JOB_NOT_FOUND has no remaining sites — it is correctly defined and available for future use