Fix admin CLI system variable commands (#14956)

## What

Fixes #12409.

Implements admin CLI support for:

- `list vars;`
- `show var <name-or-prefix>;`
- `set var <name> <value>;`

## Changes

- Wire Go CLI variable commands to the admin API.
- Support integer and quoted string values in `SET VAR`.
- Return variable rows as `data_type`, `name`, `setting_type`, and
`value`.
- Add exact-name lookup with prefix fallback for `SHOW VAR`.
- Validate values by stored data type: `string`, `integer`, `bool`, and
`json`.
- Keep the legacy Python admin CLI/server behavior aligned.
- Update admin CLI docs and add focused tests.

## Verification

- `go test -count=1 ./internal/cli`
- `python3.12 -m py_compile admin/server/services.py
admin/server/routes.py api/db/services/system_settings_service.py
admin/client/parser.py admin/client/ragflow_client.py`
- Python admin CLI parser smoke test for `SET VAR`, quoted values, `SHOW
VAR`, and `LIST VARS`.
- Attempted `./run_go_tests.sh`; local environment is missing native
tokenizer/linker artifacts:
  - `internal/cpp/cmake-build-release/librag_tokenizer_c_api.a`
  - `-lstdc++`

Co-authored-by: Jin Hai <haijin.chn@gmail.com>

admin/client/parser.py

@@ -264,7 +264,7 @@ generate_key: GENERATE KEY FOR USER quoted_string ";"
 list_keys: LIST KEYS OF quoted_string ";"
 drop_key: DROP KEY quoted_string OF quoted_string ";"
 
-set_variable: SET VAR identifier identifier ";"
+set_variable: SET VAR identifier variable_value ";"
 show_variable: SHOW VAR identifier ";"
 list_variables: LIST VARS ";"
 list_configs: LIST CONFIGS ";"
@@ -378,6 +378,7 @@ update_chunk: UPDATE CHUNK quoted_string OF DATASET quoted_string SET quoted_str
 identifier_list: identifier (COMMA identifier)*
 
 identifier: WORD
+variable_value: WORD | NUMBER | QUOTED_STRING
 quoted_string: QUOTED_STRING
 status: ON | WORD