Session classes should raise runtime errors instead of hiding attributes when Fluent server exits

**Current behavior**
In the PyFluent session classes, several methods and properties (e.g., those requiring an active Fluent connection) are **hidden or become inaccessible** once the Fluent server exits or the session becomes disconnected. This results in an unstable public API, makes introspection difficult, and can cause unexpected `AttributeError` issues.

**Problem**
Dynamically removing or hiding attributes based on connection state causes:

* Confusing errors (`AttributeError` instead of a clear connection-related exception)
* Unpredictable API surface, making documentation, typing, and IDE support harder
* Subtle bugs when tooling (e.g., Sphinx Gallery, auto-reset hooks) tries to inspect session objects

**Proposed improvement**
Keep the full API available at all times, but **raise a clear and consistent runtime exception** (e.g., `NotConnectedError`, `RuntimeError`, etc.) when a property or method is accessed and the session is not connected.

This provides:

* A stable, inspectable session interface
* More informative and debuggable exception messages
* Better compatibility with automated tools and Sphinx extension hooks

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Start by tracing the PyFluent session classes and the methods and properties whose availability changes after the Fluent server exits. Examine how disconnected sessions interact with Sphinx Gallery and auto-reset hooks, then define the consistent runtime exception behavior and verify that the full session API remains inspectable while disconnected.

Written by the indexing model from the issue text.