fix(activity): do not mutate caller-provided extra dict in LoggerAdapter

[1fanwang]

What was changed

temporalio.activity._ContextLoggerAdapter.process retrieves the caller's extra dict from kwargs and writes temporal_activity / activity_info keys directly into it before forwarding to the wrapped logger. When the caller reuses that same dict elsewhere, Temporal's injected keys leak into their state.

The fix is extra = dict(kwargs.get("extra") or {}) — copy once before mutating. The sibling workflow.py LoggerAdapter already does this correctly via _build_log_context.

Why

Caller-side reproducer:

shared = {"caller": "value"}
activity.logger.info("step", extra=shared)
assert shared == {"caller": "value"}  # fails on main, passes with this PR

A user found this in production where their extra dict was a long-lived per-request bag of context that downstream code re-emitted to a separate log sink — Temporal's keys ended up in every other log line for that request.

Tested

New parametrized regression test_activity_logger_does_not_mutate_caller_extra in tests/testing/test_activity.py. Asserts the caller dict is byte-equal to its pre-call state and that the logged record still has the Temporal keys. Fails on main with AssertionError (the dict has temporal_activity and activity_info injected); passes with this PR.

pytest tests/testing/test_activity.py -xvs
... 7 passed ...
ruff check / format — clean
pyright temporalio/activity.py — clean

Fixes #503

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