ensure deployment name does not contain #

[abrarsheikh]

Why are these changes needed?

Currently, # is allowed in deployment names, but it is also used as a delimiter when recovering from a checkpoint to infer the app, deployment, and replica. This leads to ambiguity and potential issues during recovery.

To resolve this, we are now prohibiting # in deployment names.

This change introduces an incompatibility for users who have already used # in their deployment names but do not use the checkpointing feature. I suspect this group is mostly tinkerers rather than production users, but I'm interested in hearing the reviewers' thoughts.

An alternative approach would be to escape # during checkpoint serialization and unescape it during deserialization. However, this felt like unnecessary complexity for limited benefit. That said, I'm open to revisiting this if reviewers prefer that approach.

Related Issue

Fixes #48260

Checks

• Added unit tests.

[edoakes]

@abrarsheikh we need to be careful making such breaking changes. I would suggest adding a clear warning that the character will be banned in a future release, leaving that in the code for 1-2 releases, then making the breaking change.

[edoakes]

Suggested change

	f"Name cannot contain '#' character, it will be removed in future releases. "
	f"Current name: {name}."
	f"Deployment names cannot contain '#' character, this will raise an error in a future release. "
	f"Current name: {name}."

[edoakes]

just making it more precise (users might not know what "name" this is referring to)