Give a clean error for invalid initial cycle point syntax

[Sanjays2402]

Closes #7390

Some malformed initial cycle point values (e.g. T, T, T00, T18) make the isodatetime parser fail with a bare ValueError: too many values to unpack instead of an ISO8601SyntaxError. _parse_iso_cycle_point only catches IsodatetimeError, so that escaped as a traceback rather than a clean WorkflowConfigError. ingest_time now converts an unrecognised ValueError from the parser into an ISO8601SyntaxError, which the existing error handling already reports readably.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[hjoliver]

@Sanjays2402 - thanks for this - your change looks good, but it doesn't fix the referenced issue because it needs to be done elsewhere in the code base as well.

Example, validation of this test workflow:

[scheduling]
    initial cycle point = T, T
    [[graph]]
        R1 = foo
[runtime]
    [[foo]]

On this PR branch I get still get the traceback:

$ cylc val ./test/
Traceback (most recent call last):
  File "/home/oliverh/cylc/cylc-flow/venv/bin/cylc", line 6, in <module>
    sys.exit(main())
             ^^^^^^
  File "/home/oliverh/cylc/cylc-flow/cylc/flow/scripts/cylc.py", line 651, in main
    ret = _main(opts, cmd_args)
          ^^^^^^^^^^^^^^^^^^^^^

 ...<snip>...

  File "/home/oliverh/cylc/cylc-flow/cylc/flow/parsec/validate.py", line 860, in coerce_cycle_point
    TimePointParser().parse(value)
  File "/home/oliverh/cylc/cylc-flow/venv/lib/python3.12/site-packages/metomi/isodatetime/parsers.py", line 240, in parse
    date_info, time_info, parsed_expr = self.get_info(timepoint_string)
                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/oliverh/cylc/cylc-flow/venv/lib/python3.12/site-packages/metomi/isodatetime/parsers.py", line 443, in get_info
    date, time_time_zone = date_time_time_zone
    ^^^^^^^^^^^^^^^^^^^^
ValueError: too many values to unpack (expected 2)