fix: set default elapsed on MockTransport responses

[SamMorrowDrums]

Summary

This PR fixes #3712 by setting a default elapsed value of timedelta(0) on responses returned from MockTransport.

Problem

When creating a Response with pre-loaded content (e.g., json=..., content=..., text=...), the content is immediately read into _content in the Response's __init__. Later, when the client wraps the response stream with BoundSyncStream/BoundAsyncStream to track elapsed time, the stream is never actually consumed because _content is already set.

This causes a RuntimeError when accessing response.elapsed:

def handler(request):
    return httpx.Response(200, json={"message": "Hello"})

transport = httpx.MockTransport(handler)
with httpx.Client(transport=transport) as client:
    response = client.get("https://example.com/")
    print(response.elapsed)  # RuntimeError: .elapsed accessed before response finished

Solution

Set elapsed = timedelta(0) as a default in MockTransport.handle_request() and handle_async_request() if the response doesn't already have _elapsed set.

This:

1. Allows users to access response.elapsed without errors
2. Preserves user-set elapsed values if they manually set it in their handler
3. Doesn't require boilerplate code in every handler (as suggested in the issue)

Changes

• Added import datetime to httpx/_transports/mock.py
• Added default elapsed setting in handle_request() (sync)
• Added default elapsed setting in handle_async_request() (async)

Testing

Verified that:

• Existing MockTransport tests pass
• response.elapsed returns timedelta(0) for pre-loaded responses
• User-set elapsed values are preserved

[SamMorrowDrums]

The CI failure appears to be an unrelated flaky network test (test_sync_proxy_close in tests/client/test_proxies.py) that timed out trying to connect to a proxy server:

E   httpx.ReadTimeout: timed out
tests/client/test_proxies.py:118

Test summary: 1 failed, 1416 passed, 1 skipped

This is not related to the MockTransport changes in this PR.

[karpetrosyan]

I'll close this for now until we have a good design decision around it