fix: avoid panic parsing non-ASCII runtime config values

[ByteBaker]

Which issue does this PR close?

• Closes panic on SET datafusion.runtime.* when value ends with non-ASCII byte (split_at char boundary) #22187

Rationale for this change

Presence of a non-ascii character in the value while setting any datafusion runtime variable was panicking.

What changes are included in this PR?

Better handling/iteration on character-boundary, and emits plan error now instead of panicking in case of such characters.

Are these changes tested?

Yes. UTs have been updated to include such cases.

Are there any user-facing changes?

No.

[ByteBaker]

Hi @alamb for your review please.

[comphead]

Thanks @ByteBaker can we please explore if runtime config can be part of SessionConfig? In SessionConfig there is already a framework that enforces type checking.

Edit: runtime configs looks looks finite according to set_runtime_variable, so should fit into main configs rather than having separate config set

[ByteBaker]

@comphead yes that'd be a lot more cleaner/robust. However, that would also be a significant change in user-facing ergonomics.

My suggestion would be to keep this PR to ensure the panic is resolved, and separately start a discussion around doing that, since that's a larger change surface (and the question if we should remove all the runtime variables from the SET xyz semantics)