docs: fix map operation examples to return JSON-serializable dict

[abhu85]

Summary

Fixes #301

The map operation documentation examples were returning BatchResult directly from Lambda handlers. Since BatchResult is not JSON serializable, these examples would cause Lambda to fail at runtime with serialization errors.

Changes

• Update all 7 handler return types from BatchResult[T] to dict
• Add .to_dict() calls to convert BatchResult before returning
• Add explanatory comments about JSON serialization requirement
• Update "Getting started" section description to reflect the dict return type
• Keep one example (check_inventory) showing how to extract and transform results from BatchResult before returning

Before

@durable_execution
def handler(event: dict, context: DurableContext) -> BatchResult[int]:
    items = [1, 2, 3, 4, 5]
    result = context.map(items, square)
    return result  # Fails: BatchResult is not JSON serializable

After

@durable_execution
def handler(event: dict, context: DurableContext) -> dict:
    """Process a list of items using map operations."""
    items = [1, 2, 3, 4, 5]
    result = context.map(items, square)
    # Convert to dict for JSON serialization (BatchResult is not JSON serializable)
    return result.to_dict()

Test plan

• Verified all 7 map operation handler examples now return dict type
• Verified all handlers use .to_dict() for BatchResult conversion
• Verified the API signature section (context.map() returns BatchResult[T]) is unchanged (correct behavior)
• Verified the "Filtering and transforming results" example shows proper result extraction pattern

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[yaythomas]

incorporated in this PR #312

I kept the author info on the commit, so you're credited @abhu85