Make all constants case-insensitive if they can be

[Erudition]

Is your feature request related to a problem? Please describe.
The models I've used often get mislead by the tool documentation about the required case of the arguments. Ex:

 "content": "Error executing tool zammad_list_ticket_priorities: 1 validation error for zammad_list_ticket_prioritiesArguments\nparams.response_format\n  Input should be 'markdown' or 'json' [type=enum, input_value='MARKDOWN', input_type=str]\n    For further information visit https://errors.pydantic.dev/2.11/v/enum",
    "id": "function-call-16147049927366973490",
    "is_error": true,
    "name": "mcp_zammad__zammad_list_ticket_priorities",
    "result": "Error executing tool zammad_list_ticket_priorities: 1 validation error for zammad_list_ticket_prioritiesArguments\nparams.response_format\n  Input should be 'markdown' or 'json' [type=enum, input_value='MARKDOWN', input_type=str]\n    For further information visit https://errors.pydantic.dev/2.11/v/enum"

Rats! That's two errors in a row, and both were due to missing the precise schema requirements for the Zammad tools. I need to be more careful about the expected enum values. It seems the expected value for the default format is lowercase `'markdown'`, not `'MARKDOWN'`.\n\nLet's try this one more time to list the priorities correctly so I can confirm the exact string for \"low\" priority before updating the tickets.\n\nI will call `zammad_list_ticket_priorities` with `response_format: \"markdown\"`.

Describe the solution you'd like
If case doesn't change the meaning, make it not matter. Robustness Principle.

Describe alternatives you've considered
Listing every caveat in the system prompt.