fix(cli): validate --repo filter on gitt issues list (#1061)

[RUNECTZ33]

Fixes #1061.

Problem

--repo <owner/name> on gitt issues list (added in #910) compares raw user input directly against repository_full_name:

issues = [
    i for i in issues
    if i.get('repository_full_name', '').lower() == repo_filter.lower()
]

So:

• Malformed inputs like ownerrepo or owner//repo are silently accepted and quietly match nothing (no error feedback).
• Whitespace-padded valid inputs like ' entrius/gittensor ' fail to match real entries because the comparison string still contains the surrounding spaces.

Fix

Reuse the existing validate_repository(verify_exists=False) helper before any contract read. It already:

• strips whitespace,
• enforces the owner/repo regex,
• raises click.BadParameter with param_hint='--repo' on bad input.

Surface that exception through the existing handle_exception path so JSON consumers get a structured bad_parameter error and human callers exit non-zero — same shape used by --id validation a few lines above.

The same validator is already used by mutating commands (gitt issues register/cancel in mutations.py), so the read-side --repo filter now matches the documented contract enforced everywhere else in the CLI.

Tests added

6 new tests in tests/cli/test_issues_list_json.py:

• Malformed filters rejected with structured bad_parameter error in JSON mode (parametrized over ownerrepo, owner//repo, owner/, /repo, owner repo).
• Malformed filters exit non-zero in human mode.
• Whitespace-padded valid filter ' owner/repo ' correctly matches contract entries.
• Mixed-case filter OWNER/REPO still matches (preserves existing case-insensitive behavior).
• Valid non-matching filter returns empty list, not all issues.

All tests pass; read_issues_from_contract is asserted not-called for invalid input so we never burn a contract read on bad parameters.

tests/cli/test_issues_list_json.py ... 19 passed in 0.09s

Diff size

• gittensor/cli/issue_commands/view.py: +12 lines (1 import + 8-line validation block + 1-line comment)
• tests/cli/test_issues_list_json.py: +95 lines (6 new tests)