fix regression in aws-smithy-mocks MatchAny mode and add a new repeatedly()...

fix regression in aws-smithy-mocks MatchAny mode and add a new repeatedly() function for infinite sequences (#4136)

## Motivation and Context
fixes https://github.com/smithy-lang/smithy-rs/issues/4135

## Description
* **Fixed regression in RuleMode::MatchAny**: Simple rules (created with
`.then_output()`, `.then_error()`, etc.) now correctly work repeatedly
in `MatchAny` mode, restoring backward
compatibility
* **Added repeatedly() method**: New API to make the last response in a
sequence repeat indefinitely
* **Improved times(n) implementation**: Now uses repetition counts
instead of cloning generators, improving memory efficiency
* **Added type state pattern**: Prevents adding more responses after
calling `repeatedly()`
* **Added validation**: Better error messages when `times(n)` is misused
* **Fixed overflow handling**: Used saturating_add to safely calculate
total responses
* **Added unit tests**: New tests for simple rules, `repeatedly()`, and
regression
* **Updated documentation**: Added examples and explanations for the new
functionality

## Questions

1. I've added a typestate pattern for `repeatedly()` since adding new
outputs to such a rule would never be used/invalid. Not sure if this is
worth it or not though.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.

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