Derive Default on generated Rust types (#1272)

* Derive Default on generated Rust types

When the @github/copilot schema gains an optional field on a wire type,
every Rust test fixture that constructs that type by struct literal
breaks until each one is hand-edited (cf.
70eb60e in #1270). Test fixtures
shouldn't have to acknowledge new wire fields they don't care about.

Teach the Rust codegen to derive `Default` on every generated struct
and string-style enum (with `#[default]` on the `Unknown` catch-all
where one exists), and propagate non-default-ness across structs that
have a required field of an untagged-enum type (which has no obvious
default variant). Also derive `Default` on `RequestId` for parity with
`SessionId`. With this in place, the three internal Model fixtures can
spell themselves with `..Default::default()` and survive future
additive schema changes without manual intervention.

Production code stays explicit: the rust-coding skill already forbids
`..Default::default()` outside tests, and that rule is unchanged.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Drop redundant model_picker fields after merge

main added explicit None initializers for the new model_picker_*
fields; the merge layered those on top of our ..Default::default()
making them redundant.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Regenerate after merging main

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: tclem