Wrap up low level placeholder handling into PlaceholderConfig helper

[ianjosephwilson]

• Move low level placeholder operations and constants into placeholder config helper class.
• This is to avoid potential free-threading issues but overall we probably shouldn't be using dynamic module level variables anyways.
• Long term we probably want to share the placeholder configuration across a "group" of parsers but I think this is ok for now.

[pauleveritt]

Thanks for doing this. Once this is merged, I'll switch my aria-testing package (which I use heavily in other packages) to free threading and see if I spot any problems. I'll also put the pytest-freethreaded stuff in that package and keep trying to trigger.

As a note, the only potential (but unlikely) other issue CC found:

🟡 POTENTIAL ISSUE: Mutable Dataclass Children (nodes.py, parser.py)

@dataclass
class OpenTElement:
    tag: str
    attrs: tuple[TAttribute, ...]
    children: list[TNode] = field(default_factory=list)  # Mutable!

Problem: list is mutable. If multiple threads access/modify the same OpenTElement's children, race conditions occur.

Current Safety: These are created per-parse operation and not shared between threads, so likely safe in practice.

Impact: LOW - Only matters if users share parser objects across threads (discouraged pattern)

[ianjosephwilson]

Yeah I don't think there is any use-case where parsing the same template across threads or even asyncio workers would make any sense. We can just ignore that for the foreseeable future. I'm not even sure the use-case for free threading but it will probably help avoid regular threading issues at the same time. We might have to do something to protect whatever cache we end up using from getting stampeded on startup but I think that's for later.

[pauleveritt]

Apologies, I should have framed this better. It's less about making tdom take advantage of threads and more about getting it marked as safe in a ft application. 😉 But I'm ok with just closing this PR if you'd like.

[davepeck]

@ianjosephwilson Thanks! I'm back and starting to look over tdom stuff, although it's likely not until later this week when I'll have time to really dig in. This is great, though; merging.