git: reject remote names invalid in fetch refspecs

[lawrence3699]

Fixes #9099

jj git remote add currently panics if the symbolic remote name can't be used in the generated fetch refspec.
For example, jj git remote add 'this also fails' https://github.com/jj-vcs/jj.git aborts with ReferenceName(InvalidByte { byte: " " }) instead of returning a user-facing error.

This validates symbolic remote names with gix::remote::name::validated() before writing the remote config, so invalid names fail cleanly instead of panicking.
I also added CLI coverage for empty, whitespace-containing, and whitespace-only remote names.

Validation:

• cargo test -q -p jj-cli --test runner test_git_remote

Checklist

If applicable:

• [n/a] I have updated CHANGELOG.md
• [n/a] I have updated the documentation (README.md, docs/, demos/)
• [n/a] I have updated the config schema (cli/src/config-schema.json)
• I have added/updated tests to cover my changes
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments.

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[PhilipMetzger]

Can you please move the PR description into the commit message to adhere to our style, thanks.

[yuja]

Dup of #9155?