Skip default evaluation and callbacks during resilient parsing

[barry3406]

During shell completion, Context.resilient_parsing is set to True.
Its documentation states:

if this flag is enabled then Click will parse without any interactivity
or callback invocation. Default values will also be ignored.

However, Parameter.consume_value still evaluated a default= callable
and Parameter.process_value still invoked the parameter's callback
while completion was running. This produced unwanted side effects during
tab-completion for callers that pass an expensive callable as default
or attach a callback with side effects.

Now both paths check ctx.resilient_parsing and skip the default/callback,
matching the documented behavior. The _resolve_context docstring in
shell_completion.py already said "it doesn't trigger input prompts or
callbacks" - this brings the implementation in line.

fixes #2614

Changes

• Parameter.consume_value: skip self.get_default(ctx) when
  ctx.resilient_parsing is True.
• Parameter.process_value: skip self.callback invocation when
  ctx.resilient_parsing is True.
• Option.process_value: same skip for the bool-flag early-return
  branch.
• Three tests in tests/test_shell_completion.py covering default=
  callables, parameter callback, and bool flag callback.
• CHANGES.rst entry added under Unreleased.