Incorrect invalid YAML handling in receptionist

Current situation

A malformed YAML workflow causes the receptionist service to generate a stack trace:

jobs:
    launchGenerator:
      generator: tm.squashtest.org/tm.generator@v1
      runs-on:
      - linux
      with:
        squashTMAutomatedServerLogin: tfserver
        squashTMAutomatedServerPassword: tfserver
        squashTMUrl: http://nightly-mariadb-sqtm.nightly-mariadb:8080
        testPlanType: Iteration
        testPlanUuid: ae7ae739-5409-45c5-83d4-d4b8f730261d
kind: Workflow
    name: Workflow avec generator
    namespace: default

(Note the missing metadata: item between kind: and name:.)

The generated log is:

[2024-03-08 11:17:37,063] INFO in eventbus: 127.0.0.1 - - "GET /subscriptions HTTP/1.1" 200 99486 "-" "python-requests/2.31.0"
[2024-03-08 11:17:42,146] ERROR in receptionist: Exception on /workflows [POST]
Traceback (most recent call last):
  File "/usr/local/lib/python3.12/site-packages/flask/app.py", line 2190, in wsgi_app
    response = self.full_dispatch_request()
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/flask/app.py", line 1486, in full_dispatch_request
    rv = self.handle_user_exception(e)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/flask/app.py", line 1484, in full_dispatch_request
    rv = self.dispatch_request()
         ^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/flask/app.py", line 1469, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/opentf/commons/__init__.py", line 285, in wrapper
    return function(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/opentf/core/receptionist.py", line 238, in handle_workflow
    workflow = _get_workflow()
               ^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/opentf/core/receptionist.py", line 145, in _get_workflow
    return yaml.safe_load(request.data)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/yaml/__init__.py", line 125, in safe_load
    return load(stream, SafeLoader)
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/yaml/__init__.py", line 81, in load
    return loader.get_single_data()
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/yaml/constructor.py", line 49, in get_single_data
    node = self.get_single_node()
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/yaml/composer.py", line 36, in get_single_node
    document = self.compose_document()
               ^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/yaml/composer.py", line 55, in compose_document
    node = self.compose_node(None, None)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/yaml/composer.py", line 84, in compose_node
    node = self.compose_mapping_node(anchor)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/yaml/composer.py", line 127, in compose_mapping_node
    while not self.check_event(MappingEndEvent):
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/yaml/parser.py", line 98, in check_event
    self.current_event = self.state()
                         ^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/yaml/parser.py", line 428, in parse_block_mapping_key
    if self.check_token(KeyToken):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/yaml/scanner.py", line 116, in check_token
    self.fetch_more_tokens()
  File "/usr/local/lib/python3.12/site-packages/yaml/scanner.py", line 223, in fetch_more_tokens
    return self.fetch_value()
           ^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/yaml/scanner.py", line 577, in fetch_value
    raise ScannerError(None, None,
yaml.scanner.ScannerError: mapping values are not allowed here
  in "<byte string>", line 13, column 9:
        name: Workflow avec generator
            ^