test: convert test_integration.py to in-memory transport

The file previously spun up a uvicorn subprocess + TCP port for each of
20 parametrized tests (10 tests × 2 transports). This caused flaky CI
failures from port races under pytest-xdist: the `server_port` fixture
releases its socket before the subprocess binds, so another xdist worker
can grab the same port first. `wait_for_server` only checks for TCP
connect, so it happily accepts a connection to the WRONG server.

None of these tests exercise transport-specific behavior — they test MCP
protocol features (tools, resources, prompts, progress, sampling,
elicitation, notifications, completion, structured output). The transport
parametrization doubled test time without adding coverage; transport
behavior is tested separately in tests/server/test_sse_security.py,
test_streamable_http_{manager,security}.py, and tests/shared/.

Converted all tests to use `Client(mcp)` in-memory, matching the
established pattern in test_server.py and test_url_elicitation.py.

Impact:
- Eliminates port race flakiness for this file
- 20 tests → 10 tests (no transport parametrization)
- ~20s subprocess spawn overhead → 0.5s total
- -284 lines (deleted subprocess/fixture infrastructure)

Github-Issue: #1776

Author: maxisbey