agenda file restrictions no longer work with org-ql-block

### OS/platform

Linux (Ubuntu 24.04)

### Emacs version and provenance

Emacs 30

### Emacs command

emacs

### Org version and provenance

9.7.39

### org-ql package version and provenance

commit 4b8330a

### Actions taken

1) Define a custom agenda view, for example the same one as in the README

```
(setq org-agenda-custom-commands
'(("ces" "Custom: Agenda and Emacs SOMEDAY [#A] items"
((org-ql-block '(and (todo "SOMEDAY")
(tags "Emacs")
(priority "A"))
((org-ql-block-header "SOMEDAY :Emacs: High-priority")))
(agenda)))))
```

2) Switch buffer to any org buffer

3) Run that agenda view with the agenda restricted to that file

### Observed results

There is an error

### Expected results

Not to be an error

### Backtrace

```elisp
Debugger entered--Lisp error: (wrong-type-argument stringp t)
org-ql-block((and (todo "SOMEDAY") (tags "Emacs") (priority "A")))
#f(compiled-function () #)()
funcall(#f(compiled-function () #))
(let ((org-ql-block-header '#("SOMEDAY :Emacs: High-priority" 0 29 (face org-agenda-structure)))) (funcall '#f(compiled-function () #)))
eval((let ((org-ql-block-header '#("SOMEDAY :Emacs: High-priority" 0 29 (face org-agenda-structure)))) (funcall '#f(compiled-function () #))))
org-agenda-run-series("Custom: Agenda and Emacs SOMEDAY [#A] items" (((org-ql-block '(and (todo "SOMEDAY") (tags "Emacs") (priority "A")) ((org-ql-block-header #("SOMEDAY :Emacs: High-priority" 0 29 (face org-agenda-structure))))) (agenda))))
#(nil)
apply(# nil)
org-agenda(nil)
funcall-interactively(org-agenda nil)
command-execute(org-agenda)
```

### Etc.

I debugged the issue and I know what happens, that's why I didn't bother to fill in properly the detailed versions information. This is the timeline of the bug:

Originally I assume that agenda file restrictions worked, but subtree restrictions didn't, so https://github.com/alphapapa/org-ql/issues/84 was reported an it was fixed with https://github.com/alphapapa/org-ql/commit/0f5cf5eff320cd718d60359d4562430955719040. I assume that at the time, file restrictions kept working.

That fix relies on `org-agenda-restrict` value being a buffer, since as we can see the code does `(with-current-buffer org-agenda-restrict ...)`

But if we see the `org-agenda-restrict` documentation, it says:
```
Non-nil means agenda restriction is active.
This is an internal flag indicating either temporary or extended
agenda restriction. Specifically, it is set to t if the agenda
is restricted to an entire file, and is set to the corresponding
buffer if the agenda is restricted to a part of a file, e.g. a
region or a subtree. In the latter case,
‘org-agenda-restrict-begin’ and ‘org-agenda-restrict-end’ are set
to the beginning and the end of the part.
```

So I guess at some point the possible values of that variable changed and the value t started being allowed, probably in commit https://cgit.git.savannah.gnu.org/cgit/org-mode.git/commit/?id=c57b03032380de519bafcf903ca9236668038d4e

I guess that since then file restrictions are broken in org-ql

I reverted that commit and then file restriction works (but then subtree restriction doesn't work again)