nit: Add pagination bounds to prevent DoS via unbounded queries

[smoreinis]

Summary

• Add Query validation to spans.py: limit (1-1000), page_number (>= 1)
• Add Query validation to agent_api_keys.py: limit (1-1000), page_number (>= 1)

Problem

Endpoints accepted unbounded limit parameters, allowing requests like ?limit=10000000 that could exhaust server memory.

Test Results

Test	Result
limit=10000	❌ Rejected (422): "Input should be less than or equal to 1000"
limit=0	❌ Rejected (422): "Input should be greater than or equal to 1"
page_number=0	❌ Rejected (422): "Input should be greater than or equal to 1"
limit=100	✅ Success

How to Verify

# Should return 422 error
curl "http://localhost:5003/spans?limit=10000"

# Should succeed
curl "http://localhost:5003/spans?limit=100"

Files Changed

• src/api/routes/spans.py
• src/api/routes/agent_api_keys.py