lp_diag parsing error

### What happened?

When running lp_diag on my mps file the parser fails.

I get this type of parser fail

example of the line in the mps file
`"G COMMODITY_BALANCE_GT(R12_AFR,'Fertilizer Use|Nitrogen',final_material,2035,year)"`

### Code Sample

```python
in __init__ inside message_ix\tools\lp_diag
ln 248

in add_row
assert n_words == 2, (
^^^^^^^^^^^^
AssertionError: row declaration (line 7917) has 3 words instead of 2.
```

### What did you expect to happen?

example of the line in the mps file
`"G COMMODITY_BALANCE_GT(R12_AFR,'Fertilizer Use|Nitrogen',final_material,2035,year)"`

### Versions

```shell

```

### Additional Context

_No response_

Start in message_ix/tools/lp_diag, reviewing __init__ around line 248 and add_row where the assertion reports line 7917. Reproduce the failure with the shown MPS row and determine how its quoted declaration is expected to be parsed. Done means this input no longer triggers the row-declaration assertion and its parsing behavior is covered by an appropriate regression check.

Written by the indexing model from the issue text.