Add response delay simulation for testing request timeouts #2884

Summary

Add support for simulating slow HTTP responses to enable testing of request timeout handling in client code.

Proposed Interface

from mock_vws import MockVWS

# Simulate 0.5 second delay on all responses
with MockVWS(response_delay_seconds=0.5) as mock:
    database = VuforiaDatabase()
    mock.add_database(database=database)
    
    # Client with short timeout will fail
    vws_client = VWS(
        server_access_key=database.server_access_key,
        server_secret_key=database.server_secret_key,
        request_timeout_seconds=0.1,
    )
    
    vws_client.list_targets()  # Raises requests.exceptions.Timeout

Use Case

vws-python recently added configurable request_timeout_seconds to VWS and CloudRecoService. To properly test this, we need to verify that:

1. Short timeouts raise requests.exceptions.Timeout when the server is slow
2. Longer timeouts allow slow responses to complete

Currently, we're using unittest.mock.patch to intercept requests.request and artificially raise Timeout, but this:

• Doesn't test the real timeout mechanism
• Requires ugly type annotation hacks
• Tests at the wrong layer

With response_delay_seconds, we could test the actual integration end-to-end.

Expected Behavior

• response_delay_seconds defaults to 0 (no delay)
• When set, all mock responses are delayed by the specified number of seconds
• The delay happens at the HTTP response level so that requests' native timeout handling triggers naturally