ReFrame crashes if a partition name with `:` is defined

[vkarak]

Example configuration file:

systems:
- name: tresa
  descr: My Mac
  hostnames:
  - tresa
  partitions:
  - name: default:foo
    scheduler: local
    launcher: local
    container_platforms:
    - type: docker
    environs:
    - gnu
    - clang
    - builtin
    max_jobs: 1000
environments:
- name: gnu
  cc: gcc-15
  cxx: g++-15
  ftn: gfortran-15
  features:
  - openmp
  extras:
    omp_flag: -fopenmp
  target_systems:
  - tresa
- name: clang
  cc: clang
  cxx: clang

Run with

uv run reframe -C config/machines.yaml -c examples/tutorial/stream/stream_build_run.py -l

ERROR: run session stopped: value error: too many values to unpack (expected 2, got 3)
ERROR: Traceback (most recent call last):
  File "/Users/karakasv/Repositories/reframe/reframe/frontend/cli.py", line 1400, in main
    testcases_all = generate_testcases(checks_found)
  File "/Users/karakasv/Repositories/reframe/reframe/core/logging.py", line 1138, in _fn
    return fn(*args, **kwargs)
  File "/Users/karakasv/Repositories/reframe/reframe/frontend/executors/__init__.py", line 191, in generate_testcases
    valid_comb = runtime.valid_sysenv_comb(c.valid_systems,
                                           c.valid_prog_environs)
  File "/Users/karakasv/Repositories/reframe/reframe/core/runtime.py", line 399, in valid_sysenv_comb
    if check_systems and not _is_valid_part(part, valid_systems):
                             ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/Users/karakasv/Repositories/reframe/reframe/core/runtime.py", line 295, in _is_valid_part
    sysname, partname = part.fullname.split(':')
    ^^^^^^^^^^^^^^^^^
ValueError: too many values to unpack (expected 2, got 3)

Names with : should not be allowed.