fix: prevent print-job entities from showing as unavailable when idle

[BenHornerTech]

Description

Fixes several entities becoming unavailable in Home Assistant when the printer is idle or a print finishes. Entities now remain available at all times, with safety guards added to prevent commands being sent when the printer isn't in the correct state.

Buttons (Pause / Resume / Stop) - button.py:

• Remove available() overrides so buttons inherit CoordinatorEntity.available (still correctly returns False when MQTT/connectivity is lost)
• Add state guards inside async_press() - commands are only published when the printer is in the correct gcode state; ignored presses log a warning

Printing Speed Select - select.py:

• Remove available() override that restricted the select to gcode_state == 'RUNNING'
• Add state guard inside async_select_option() checking for RUNNING or PAUSE before applying speed changes

Print-job Sensors - definitions.py:

• start_time: Set available_fn to always return True; existing _start_time_value_fn() already safely returns None
• end_time: Set available_fn to always return True; add null guard to value_fn to prevent dt_util.as_utc(None) raising an AttributeError
• gcode_file / gcode_file_downloaded: Set available_fn to always return True; return None instead of empty string when no file is set

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Other (please describe):

Documentation

• I have updated the relevant documentation to reflect these changes
• No documentation updates were necessary for this change

Testing

• I have added/updated tests that prove my fix is effective or that my feature works
• All new and existing tests passed

Additional Notes

Claude was used to assist with these fixes and I have tested on my P1S with AMS. I verified the entities remain available when idle, show unavailable only when the printer is genuinely offline, and buttons/speed select still function correctly during a print.

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