Use tcp sensor available flag to reflect connection errors

[fopina]

Breaking change

• before: when a connection fails, there is no state change - it remains with the value from the last successful connection
• now: when a connection fails, state is updated to unavailable

This might have impact on automations / triggers as unavailable state was not used before, not sure if that's considered breaking change.

Proposed change

Currently when a TCP sensor fails to connect (DNS failure, port is closed, host is not reachable or connection times out), state does not reflect it.
It continues to show the last successful connection which is not accurate.

I propose to use sensor available flag by setting it to False whenever a connection exception occurs

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
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Checklist

• The code change is tested and works locally.
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[fopina]

Added new test to assert available flag changes (that I have validated using the component in my instance) but unable to find a quick way to retrieve the entity and assert it.
Is there an helper for it? Or another way?

[fopina]

Isn't there a constant for it? I only find multiple defined at specific component levels