ait-yaml-validate does not schema-check !include'd dictionary files (#201 still reproduces)

Start with YAMLProcessor and schema_val() in ait/core/val.py:100-104 and :495-503, then read the Cmd/Tlm validation paths at :587 and :715 and tests/ait/core/test_val.py. Reproduce the included command and telemetry cases from the issue; done means schema errors in included files fail validation while shipped configuration and clean includes still pass.

ait-yaml-validate --cmd and --tlm don't run the JSON schema over files pulled in with !include, so a misspelled key or wrong-typed value in an included command or telemetry dictionary still ends in Validation: SUCCESS, exit 0. #201 reported this and #452 closed it, but on master (a01414c, 3.1.2) the schema check still skips includes.

schema_val() checks only the top-level file (ait/core/val.py:503), where YAMLProcessor has already turned - !include ccsds_header.yaml into - include: line 96 (val.py:100-104), which the schema accepts. The loop over yml_files_to_validate (the top-level file plus its includes, val.py:495-497) logs Validating: <file> but only runs content_val(), and the Cmd/Tlm versions load CmdDict(self._ymlfile) / TlmDict(self._ymlfile) regardless of which file the loop passes in (val.py:587, :715).

From the repo root, with AIT_CONFIG set:

cp -r config cfg
printf -- '- !Command\n  name: X\n  opcode: 0x0099\n  subsytem: CORE\n' > cfg/extra.yaml
printf -- '\n- !include extra.yaml\n' >> cfg/cmd.yaml
ait-yaml-validate --cmd --yaml cfg/cmd.yaml    # Validation: SUCCESS, exit 0
ait-yaml-validate --cmd --yaml cfg/extra.yaml  # 'subsytem' was unexpected, exit 1

Other cases, each on a fresh copy of config/:

• mask: renamed maks: on CCSDS_HEADER.version in the shipped ccsds_header.yaml (included at config/tlm.yaml:97): exit 0. The same edited header pasted inline: exit 1; the unedited header inline: exit 0. With the maks file, TlmDict drops the mask and decodes 08 01 c0 00 00 05 as version=8 instead of 0.
• Field includes (doc/source/telemetry_intro.rst:367): maks: in the included field list exits 0, inline exits 1.
• A duplicate opcode in an include fails (exit 1, an uncaught YAMLError from CmdDict), so includes are loaded, just not schema-checked.

The tlm_yaml_check and cmd_yaml_check pre-commit hooks (.pre-commit-config.yaml:44-56) call the same entry point (read; the hooks weren't run), and tests/ait/core/test_val.py has no include case. Not tested: EVR and limits dictionaries, includes nested more than one level. de1a0f9 (#452's merge) was read, not run; it has the same loop.

Schema-checking each included file on its own isn't enough. I tried it: it catches the command case but fails a clean Field include ('field', 'type' were unexpected). Expanding - !include lines inside YAMLProcessor.process() before the schema pass caught the command, header and field typos, passed the shipped config and clean includes, and test_val.py stayed at 26 passed. Error line numbers then refer to the expanded text. Happy to open a PR with that plus a bad-include test, or to hear you'd rather handle it another way.