PPF-1707: Add a __repr__ method to the Widget base class

[annaswims]

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What this PR does

#1711
Adds __repr__ to the Widget base class (middleware/base.py) so that widgets render meaningfully when inspected. The output includes the class name, name, value, and all non-None public attributes, while omitting internal bookkeeping fields (SET_ATTR_TRIGGER_HOOK_MAP, attr_set_tracker, hooks_to_trigger) and _-prefixed private attributes.

Before:

<PyPDFForm.lib.middleware.text.Text object at 0x10f3a2d10>

After:

Text(name='test', value=None, readonly=False, required=False, hidden=False, page_number=1, x=73.3365, y=662.692, width=232.4235, height=21.068, comb=False, multiline=False)

Changes

• PyPDFForm/lib/middleware/base.py — added __repr__ to Widget
• docs/inspect.md — new "Inspect individual form field widgets" section documenting pdf.widgets
• docs/index.md — added pprint(pdf.widgets) to the quick-look example
• tests/test_extract_middleware_attributes.py — six new tests covering Text and Checkbox repr output, attribute filtering, size propagation, and per-widget repr across a real PDF