Opt-in FunctionDefinition collection tree node (#3926)

[RonnyPfannschmidt]

Closes #3926.

Promotes the internal FunctionDefinition from a transient parametrization
helper to a real collector node in the collection tree, behind an opt-in ini
option.

collect_function_definition (tri-state string, default hidden)

Mode	Tree	Function-level markers
hidden (default)	flat, definition kept out of the tree	on the Function (legacy)
pedantic	definition node inserted	scoped to the definition; invocation owns only its callspec marks
messy	definition node inserted	transferred back onto each invocation (legacy layout), + header warning

messy is a migration stopgap for code that is not yet prepared for the
definition scope — it reproduces the old duplicated-marker layout so bad code
keeps working. The target is pedantic.

What changes

• FunctionDefinition becomes a PyCollector (no longer a Function/Item), so
  it is collected rather than run. Its collect() runs pytest_generate_tests
  and yields the invocations.
• Invocation node ids stay flat and identical across all modes
  (test_mod.py::test_x[0]), so -k, caching, --lf, xdist and reporting are
  unaffected.
• obj/instance resolution and getmodpath transparently skip an interposed
  definition node.
• Marker scoping is correct in every mode (iter_markers yields each mark once).

The default (hidden) path is behavior-preserving; the whole test suite passes
unchanged.

Known warts / follow-up TODOs

These are intentionally left as follow-ups (inline TODO(#3926) where relevant),
to keep this PR focused — happy to split into a tracking issue:

1. Item-scoped getfixtureinfo on a collector. The fixture closure must be
   computed before the invocations exist, so FunctionDefinition._generate_functions
   calls the item-scoped getfixtureinfo on the collector via a cast. Needs a
   node-level entry point for closure computation. (TODO(#3926) inline.)
2. messy mode is temporary. It exists only to unblock migration and should
   be removed once consumers move to the definition scope. Marked clearly in the
   docs and header warning.
3. Collector/item node-id collision. For non-parametrized tests the definition
   node and its sole Function child share a node id. Accepted for now; the
   structured-nodeids work will give these a better data structure.
4. originalname could now become a read-only property returning the
   definition's name (see the existing note in Function.__init__).

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