feat: add public setter methods for ClientSession callbacks

[dgenio]

What changed

• src/mcp/client/session.py: added three public methods to ClientSession:
  • set_sampling_callback(callback)
  • set_elicitation_callback(callback)
  • set_list_roots_callback(callback)
• Removed # pragma: no cover from _default_elicitation_callback (now covered by test)
• tests/client/test_list_roots_callback.py: added test_set_list_roots_callback
• tests/client/test_sampling_callback.py: added test_set_sampling_callback
• tests/client/test_elicitation_callback.py: new file with test_set_elicitation_callback

Why

ClientSession accepts callback parameters at initialization but provided no public API to update them at runtime. Any client needing to change callbacks (e.g., updating roots after a user changes working directory) was forced to mutate private attributes like _list_roots_callback directly — coupling consumers to implementation details. This was the pattern used in PrefectHQ/fastmcp#3714.

Each setter accepts callback | None. Passing None restores the default behaviour (returning an error to the server), consistent with the __init__ fallback pattern. The docstrings note that capabilities are advertised during initialize() and won't be re-negotiated by calling these setters.

How verified

uv run --frozen pytest tests/client/test_list_roots_callback.py tests/client/test_sampling_callback.py tests/client/test_elicitation_callback.py -v
# 7 passed

uv run --frozen pytest tests/client/ -q
# 188 passed, 3 skipped, 1 xfailed

uv run --frozen ruff format .   # 357 files left unchanged
uv run --frozen ruff check .    # All checks passed

uv run --frozen pyright src/mcp/client/session.py tests/client/test_elicitation_callback.py tests/client/test_list_roots_callback.py tests/client/test_sampling_callback.py
# 0 errors, 0 warnings, 0 informations

Tradeoffs / risks

• Capability mismatch: if a callback is added after initialize(), the server won't know about the capability until a new session is created. This is documented in each setter's docstring.
• logging_callback / message_handler: not included — the issue only names the three callbacks above. Can be added as a follow-up if desired.
• strict-no-cover cannot run on Windows; CI will verify on Ubuntu.

Scope notes

Changes are strictly limited to the three callbacks named in #2379. No refactoring of existing code.