Add defaults kwarg to Form.from_model for runtime prefill

[jlowin]

Field(default=...) bakes values into the model class — every render sees the same defaults. That's wrong for the common MCP case where an LLM has collected most of a form's values already and wants the user to edit a pre-filled form rather than a blank one (see PrefectHQ/fastmcp#3930). Form.from_model() now takes a defaults dict that overlays onto the model's class-level defaults at render time.

class BugReport(BaseModel):
    title: str
    severity: Literal["low", "medium", "high"] = "low"
    reproducible: bool = False

Form.from_model(
    BugReport,
    defaults={"title": "Login broken on Safari", "severity": "high", "reproducible": True},
    on_submit=CallTool("file_bug"),
)

Partial dicts are fine — missing keys fall back to the model's defaults. Unknown keys raise ValueError to catch typos. Date/time values accept either Python date/datetime/time instances or ISO strings, so FastMCP can forward raw JSON straight through. The date/time branches also now render value= at all (previously dropped it on the floor — even the class-level default never surfaced).

Also fixes a latent bug in tools/render_previews.py: the module's from __future__ import annotations was leaking through exec() and turning every preview's field annotation into a ForwardRef string, which silently broke Pydantic introspection — e.g. Literal[...] fields rendered as text inputs instead of selects. Compiling with dont_inherit=True isolates the preview.