refactor(dag): ExportFormat implements FromStr instead of shadowing it

[ApiliumDevTeam]

Last of the stack. Base is #147. Touches a public signature, which is why it is separate.

What was wrong

ExportFormat::from_str was an inherent method returning Option. It shadows std::str::FromStr::from_str and returns a different shape from it, so a caller reading ExportFormat::from_str(..) could not tell which one they were getting — and got no reason for the failure either way.

What it is now

A real FromStr with an error that carries the offending input and knows the accepted spellings.

That last part fixed a live inconsistency: the list of valid formats lived in the REST handler, where it could drift from the parser with nothing to catch it. It already had. The handler advertised "dot, mermaid, json" while the parser also accepted graphviz and md. The type owns the list now, and a test asserts the message names every spelling the parser takes.

Blast radius

One caller outside this module's own tests, migrated here. Technically a public-API change, so worth a minor version even though every consumer is in this workspace.

Verification

#[allow(clippy::should_implement_trait)] removed — no #[allow] from this series is left anywhere.

cargo clippy --workspace --all-targets -- -D warnings exit 0 · cargo fmt --all --check exit 0 · 2248 tests green.