docs: clarify format= value contract and model_validate_json pattern

[planetf1]

Closes #1273. Closes #1274.

Supersedes #1284, which attempted a code-level fix. The maintainer decision
is to document the behaviour clearly instead.

What this PR does

format= on act/aact/instruct/ainstruct constrains the backend's
token generation to JSON matching the declared Pydantic schema. The thunk's
.value is always a str — not a parsed model instance. The previous docs
only hinted at this in a terse comment; callers had to guess the parse step
and could silently introduce a cast footgun.

docs/how-to/act-and-aact.md

• Rewrites the Structured output section with a complete working example
  (schema definition → act() call → model_validate_json parse)
• Adds a callout explaining the design contract, naming the cast footgun
  explicitly (cast(Planet, result.value) passes type checking but raises
  AttributeError at runtime), and covering the return_sampling_results
  unwrap variant
• Adds a one-sentence note to the aact() section confirming the same
  contract applies there

docs/how-to/enforce-structured-output.md

• Merges instruct(format=...) and act(format=...) into a single table
  row (they share the same contract)
• Adds a cast warning callout to Pattern 2 with the return_sampling_results
  unwrap note
• Updates the "Choosing between" section to mention act(format=...) directly

Docstrings (mellea/core/base.py, mellea/stdlib/session.py, mellea/stdlib/functional.py)

• ComputedModelOutputThunk.value — expanded from "Gets the value of the
  block" to describe the JSON-string contract and the parse step
• format parameter — updated in act, aact, instruct, ainstruct
  (both session.py and functional.py) from "BaseModel for constrained
  decoding" to include the .value-is-str contract and the
  model_validate_json pattern

Why not a code fix?

A code fix (#1284) was explored and declined in review. The design intent is
that format= is a generation hint, not a typing signal: a component
declares the shape of its output (e.g. a Message component returns a
Message), and format= is a separate concern. The clean code-level path
is @generative, which returns a typed instance directly. The docs now say
this clearly.

The type-narrowing gap (#1274) remains open as a future improvement,
tracked in #1313.