Reducing of `CustomProgressIndicator` interface to a view of textual content of a UI indicator

# Description

**The current issue is dependent on this issue: #257.**

The current `CustomProgressIndicator` allows state-changing methods, such as `stop`, `cancel`, etc. It is discouraged to call these methods within an implementation and check the cancellation via the `isCanceled` method.

The `CustomProgressIndicator` interface should be reduced to a mere view of the textual content of the underlying UI indicator. The `LLMWithFeedbackCycle` component must be called within the coroutine and checked for cancellation via `ensureActive` (or methods alike).

# Definition of Done
* [ ] Reduce the `CustomProgressIndicator` interface (rename it) + delete part of the code in `LLMWithFeedbackCycle`, which uses the state-related indicator functions, e.g., `isCanceled` method (i.e. the indicator should just be a view for the UI).
* [ ] Inside `FeedbackCycleExecutionResult`, delete the `CANCELLED` state.
* [ ] Shrink the `CustomProgressIndicator` interface to only setters of textual content of the underlying UI indicator.

Resolve the dependency on issue #257, then inspect usages of CustomProgressIndicator, LLMWithFeedbackCycle, and FeedbackCycleExecutionResult. Done means the indicator exposes only textual-content setters, state-related calls and the CANCELLED result are removed, and cancellation is handled through coroutine activity checks.

Written by the indexing model from the issue text.