fix(db,routers): improve DB URLs error handling

[francescobrivio]

PR Description

In this PR I'm improving the error reporting when DB_URL env variables are missing as suggested in the "Proposed Changes" section of #46.

PR Validation

Manually tweaking BaseSQLDB.available_urls() and BaseOSDB.available_urls() I was able to generate the desired warning messages:

• SQL:

  2026-09-05 17:02:04,332 - WARNING - TaskQueueDB found but no URL connection set: please set DIRACX_DB_URL_TASKQUEUEDB env variable to enable it.
  

• OpenSearch:

  2026-09-07 18:48:02,885 - WARNING - JobParametersDB found but no URL connection set: please set DIRACX_OS_DB_JOBPARAMETERSDB env variable to enable it.
  

Additionally, manually tweaking diracx-db/src/diracx/db/__main__.py.generate_local_urls() I was able to reproduce the desired error messages:

• For "No SQL database could be initialized":

  Exception: No SQL database could be initialized, aborting. Please set the following env variables to enable it:
   DIRACX_DB_URL_JOBDB
   DIRACX_DB_URL_LOLLYGAGDB
   DIRACX_DB_URL_MYPILOTDB
   DIRACX_DB_URL_AUTHDB
   DIRACX_DB_URL_JOBDB
   DIRACX_DB_URL_JOBLOGGINGDB
   DIRACX_DB_URL_PILOTAGENTSDB
   DIRACX_DB_URL_RESOURCESTATUSDB
   DIRACX_DB_URL_SANDBOXMETADATADB
   DIRACX_DB_URL_TASKQUEUEDB
   DIRACX_DB_URL_TASKDB
  

• For "Cannot enable system…":
  • SQL:

    NotImplementedError: Cannot enable system_name='jobs' please set the following env variables to enable it:
     DIRACX_DB_URL_TASKQUEUEDB
     DIRACX_DB_URL_JOBDB
    

  • OpenSearch:

    NotImplementedError: Cannot enable system_name='jobs' please set the following env variables to enable it:
     DIRACX_OS_DB_JOBPARAMETERSDB
    

DoD

• Starting a service whose DB env variable is unset produces an error message that contains the literal environment variable name to set (e.g. DIRACX_DB_URL_JOBDB).
• Starting with no DB URLs configured at all produces an error explaining that connection URLs are provided via DIRACX_DB_URL_ / DIRACX_OS_DB_ variables.
• DBs that are discovered but unconfigured are logged (warning level) at startup instead of being silently skipped, for both SQL and OpenSearch databases.
• Behavior is covered by tests (missing single DB, no DBs at all, OpenSearch variant) if possible.
• Malformed-URL error handling is unchanged.

Closes #46

[francescobrivio]

I still need to work on the test coverage, but while I understand exactly how the tests work, I'd like to hear your feedback on the implemented fix @chaen @aldbr @fstagni. Thank you!