…enter__ in DynamicToolset

When a `DynamicToolset` factory creates a new `MCPServer` instance on each run step (the common pattern for per-user authentication), the `for_run_step` method exits the old server and then enters the new one. If `__aenter__` fails on the new server, `self._toolset` still points to it. When the outer context manager then calls `DynamicToolset.__aexit__`, it tries to exit this unentered server, triggering `ValueError: MCPServer.__aexit__ called more times than __aenter__`.

Signed-off-by: anish k <[email protected]>

…ting transition bug

Consolidate the duplicated try/except BaseException blocks in for_run_step and
__aenter__ into a single _enter_inner_toolset helper, following the project
rule to consolidate duplicated try/except and helper logic.

Also fixes a pre-existing issue in for_run_step: if the old toolsets __aexit__
raised, the finally block stored new_toolset on self without it ever being
entered, causing DynamicToolset.__aexit__ to later attempt __aexit__ on an
unentered toolset. Now the old toolset is detached before exit, and _toolset
is only set back to a toolset after its __aenter__ succeeds.

Add tests covering all three failure paths: new toolset __aenter__ failure
during for_run_step transition, outer __aenter__ failure, and old toolset
__aexit__ failure during for_run_step transition.

… bodies

Coverage failed for 3 return statements inside never-called test helper methods
because the pragma was on the prior line, not the def. Moving the marker to the
def line excludes the whole method body.