Add default --limit to show tool (#284)

# Summary
Closes #272.

# Details 
- Set explicit value in Pydantic `Field` default param and remove `None`
type
- Extend prompt with guiding words of advice

# Tests
I don't have an OpenAI key to run `task:client` but was able to
successfully use the MCP Inspector and test in Claude/Copilot/Cursor:

Claude - on asking about the default values for show:
<img width="599" height="683" alt="Screenshot 2025-08-13 at 9 48 51 AM"
src="https://github.com/user-attachments/assets/fbac84b7-56a3-4d3b-a42e-aa2413f90bfa"
/>


GitHub Copilot and Cursor had the same results and limited the data:
<img width="1101" height="360" alt="Screenshot 2025-08-13 at 10 13
58 AM"
src="https://github.com/user-attachments/assets/959522d6-83bb-4eeb-b1a5-030b4f7a028b"
/>

---------

Co-authored-by: Devon Fulcher <24593113+DevonFulcher@users.noreply.github.com>

Author: jairus-m

.changes/unreleased/Enhancement or New Feature-20250813-070837.yaml

@@ -0,0 +1,3 @@
+kind: Enhancement or New Feature
+body: Add default --limit to show tool
+time: 2025-08-13T07:08:37.699907-07:00

src/dbt_mcp/dbt_cli/tools.py

@@ -117,9 +117,7 @@ def test(
 
     def show(
         sql_query: str = Field(description=get_prompt("dbt_cli/args/sql_query")),
-        limit: int | None = Field(
-            default=None, description=get_prompt("dbt_cli/args/limit")
-        ),
+        limit: int = Field(default=5, description=get_prompt("dbt_cli/args/limit")),
     ) -> str:
         args = ["show", "--inline", sql_query, "--favor-state"]
         # This is quite crude, but it should be okay for now

src/dbt_mcp/prompts/dbt_cli/args/limit.md

@@ -1 +1 @@
-Limit the number of rows that the data platform will return. Use this in place of a `LIMIT` clause in the SQL query.
+Limit the number of rows that the data platform will return. Use this in place of a `LIMIT` clause in the SQL query. If no limit is passed, use the default of 5 to prevent returning a very large result set.