rpptx: setting `Run.text` invalidates every slide and shape handle, unlike every other setter and unlike rdocx

[hadim]

Thanks for the rpptx binding round (#141, #142): the deck can now be edited from Python end to end. One
behaviour makes it awkward: after run.text = ..., every handle to the presentation's slides, shapes,
paragraphs and runs raises StaleElementError on next use, including the shape the run belongs to. Setting a
font, a paragraph property, a shape's position or a text frame inset leaves them valid, and in rdocx
Run.text leaves paragraph and run handles valid too. A loop that rewrites the runs of one text box has to
re-navigate from Presentation after every run.

import rpptx
from pptx import Presentation
from pptx.util import Inches

p = Presentation(); s = p.slides.add_slide(p.slide_layouts[6])
tf = s.shapes.add_textbox(Inches(1), Inches(1), Inches(4), Inches(1)).text_frame
tf.paragraphs[0].add_run().text = "alpha "; tf.paragraphs[0].add_run().text = "beta"
p.save("rs.pptx")


def fresh():
    prs = rpptx.Presentation("rs.pptx"); slide = prs.slides[0]; shape = slide.shapes[0]
    return prs, slide, shape


mutations = {
    "Run.text = ...": lambda prs, sl, sh: setattr(sh.text_frame.paragraphs[0].runs[0], "text", "ALPHA "),
    "Run.font.italic = True": lambda prs, sl, sh: setattr(sh.text_frame.paragraphs[0].runs[0].font, "italic", True),
    "Paragraph.space_after = ...": lambda prs, sl, sh: setattr(sh.text_frame.paragraphs[0], "space_after", 12700),
    "Shape.left = ...": lambda prs, sl, sh: setattr(sh, "left", sh.left + 12700),
    "TextFrame.margin_left = 0": lambda prs, sl, sh: setattr(sh.text_frame, "margin_left", 0),
}
for label, fn in mutations.items():
    prs, slide, shape = fresh()
    run = shape.text_frame.paragraphs[0].runs[1]
    fn(prs, slide, shape)
    out = []
    for name, probe in (("same shape", lambda: shape.left), ("same slide", lambda: len(slide.shapes)),
                        ("second run", lambda: run.text)):
        try:
            probe(); out.append(f"{name}: valid")
        except Exception as e:
            out.append(f"{name}: {type(e).__name__}")
    print(f"{label:<30} " + " | ".join(out))

Run.text = ...                 same shape: StaleElementError | same slide: StaleElementError | second run: StaleElementError
Run.font.italic = True         same shape: valid | same slide: valid | second run: valid
Paragraph.space_after = ...    same shape: valid | same slide: valid | second run: valid
Shape.left = ...               same shape: valid | same slide: valid | second run: valid
TextFrame.margin_left = 0      same shape: valid | same slide: valid | second run: valid

The README documents stale handles "after structural changes"; replacing the text of an existing run does
not change the structure: TextRunMut::set_text (rpptx/src/lib.rs:7189) only sets the run's text, and
the revision is bumped by the binding (rpptx-py/src/text.rs:886). PR #149 records it ("a later
run.text = ... also advances the revision") next to the rule that in-place setters do not advance it. Could the setter keep handles valid, as
the font setters do?

Environment: main at 9a7ed714 (S75), release build, linux x86_64, python-pptx 1.0.2 for the fixture.