Improve error messages for incorrect Depends() usage

Description

This PR improves the developer experience when Depends() is used incorrectly by providing clear, actionable error messages instead of cryptic TypeErrors from deep in the stack.

Problem

When new FastAPI users misuse the Depends() function, they encounter confusing error messages like:

• TypeError: 'dict' object is not callable
• AttributeError: 'Depends' object has no attribute...

These errors come from Starlette or Pydantic internals and don't clearly explain what went wrong or how to fix it.

Solution

Added validation in the Depends.__init__() method to catch common mistakes early and provide helpful error messages.

Common mistakes now caught:

1. Calling the dependency function:

# ❌ Wrong
@app.get("/")
def route(user = Depends(get_user())):
    pass

# ✓ Correct
@app.get("/")
def route(user = Depends(get_user)):
    pass

2. Nesting Depends() calls:

# ❌ Wrong
@app.get("/")
def route(val = Depends(Depends(get_value))):
    pass

3. Passing non-callable values:

# ❌ Wrong
@app.get("/")
def route(val = Depends("not_a_function")):
    pass

Changes

Modified fastapi/params.py:

• Added validation logic in Depends.__init__()
• Checks for nested Depends() calls
• Checks if dependency is callable
• Provides specific error messages with examples

Added tests/test_depends_validation.py:

• Tests for all error cases (nested Depends, called functions, non-callables)
• Tests for correct usage still working
• Tests for error message quality and helpfulness
• Tests for callable classes and edge cases

Example Error Messages

Before:

TypeError: 'dict' object is not callable

After:

TypeError: Depends() expects a callable (function or class), but received dict.

It looks like you may have called the dependency function instead of passing it.
✓ Correct: Depends(my_function)
✗ Wrong: Depends(my_function())

Benefits

• ✅ Fails fast - Catches errors at route definition time, not runtime
• ✅ Clear guidance - Shows exactly what's wrong and how to fix it
• ✅ Minimal change - Only modifies one method in fastapi/params.py
• ✅ No breaking changes - Correct usage is completely unaffected
• ✅ Well tested - Comprehensive test coverage for all scenarios
• ✅ Improves DX - Significantly better experience for new users

Testing

• Added comprehensive test coverage (tests/test_depends_validation.py)
• All existing tests pass
• Manual testing confirms improved error messages
• Code formatted with Black and isort
• Type checking passes

Checklist

• Clear, actionable error messages with examples
• Catches errors at route definition time (early)
• Maintains backward compatibility (correct usage unaffected)
• Includes helpful examples in error messages (✓/✗ format)
• Comprehensive test coverage (8 test cases)
• Follows FastAPI code style and conventions

Related Issues

This addresses a common pain point for new users learning FastAPI's dependency injection system. Related discussions:

• Users encountering cryptic Depends() errors in community channels
• Multiple Stack Overflow questions about these error messages

This enhancement aligns with FastAPI's philosophy of excellent developer experience and will help onboard new users more smoothly.