BE-859: Connect ProblemType and ProblemDetails

[TimDiekmann]

🌟 What is the purpose of this PR?

Create problem occurrences from a shared ProblemType through calls such as INVALID_PARAMETERS.detail(message).instance(uri). These calls borrow the type URI and title from the definition, and callers can omit extensions without constructing NoExtensions.

🔗 Related links

• BE-859: Connect ProblemType and ProblemDetails (internal)

Ignore BE-353

🚫 Blocked by

• BE-853: Add the initial problematic crate #9683 — the initial problematic crate; this PR is based on its branch.

🔍 What does this change?

• Add const conversions from &ProblemType and ProblemType to ProblemDetails, with empty extensions and omitted occurrence fields. Converting a reference borrows the metadata; converting an owned definition moves it.
• Add .detail(), .instance(), and .extensions() to both types. Each call returns complete problem details, and replacing extensions can change their type.
• Default the extension type to NoExtensions.
• Support const construction through nightly const traits. The destruction bound applies to the old extension value when it is replaced; runtime calls support extensions with ordinary Drop implementations.
• Document the methods with complete examples for borrowed explanations, owned instance URIs, and serialized extension members.
• Use the fluent methods in existing serialization tests and check borrowed and owned strings with assert_matches!.

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• modifies a Cargo-publishable library, but it is not yet ready to publish

📜 Does this require a change to the docs?

The changes in this PR:

• require changes to docs which are made as part of this PR

🕸️ Does this require a change to the Turbo Graph?

The changes in this PR:

• do not affect the execution graph

⚠️ Known issues

The crate requires nightly features const_convert, const_destruct, and const_trait_impl. Const callers use Cow::Borrowed(...) for string literals and enable const_convert and const_trait_impl. Runtime calls accept &str, String, and Cow.

🐾 Next steps

Connect problem definitions to OpenAPI and the public /v1 API in follow-up work.

🛡 What tests cover this?

• Five doctests cover runtime and const construction, borrowed explanations, owned instance URIs, and serialized extension members.
• A source-local unit test verifies that converting a reference borrows the definition's owned metadata strings.
• The details_static_metadata integration test creates an occurrence through fluent calls, checks that the detail and instance borrow the original strings, and checks the serialized output.
• All 30 crate tests pass, including extension-validation tests and JSON Schema snapshots. All five doctests, Clippy, Rustdoc, and formatting checks also pass.

❓ How to test this?

Run from the repository root with the pinned toolchain:

cargo nextest run -p problematic --all-features
cargo test -p problematic --all-features --doc
cargo clippy -p problematic --all-features --all-targets --no-deps -- -D warnings
RUSTDOCFLAGS='-D warnings' cargo doc -p problematic --all-features --no-deps
cargo fmt -p problematic --check

📹 Demo

let details = INVALID_PARAMETERS
    .detail("The limit must be a positive integer.")
    .instance("/problem-occurrences/42");

let details = details.extensions(extensions);

let details = ProblemDetails::from(&INVALID_PARAMETERS);

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[cursor]

PR Summary

Low Risk
Library-only API ergonomics for HTTP problem details; no auth, persistence, or wire-format behavior changes beyond optional default type parameter.

Overview
Connects shared ProblemType definitions to RFC 9457 occurrences via From conversions and a fluent builder API, so callers can write INVALID_PARAMETERS.detail(msg).instance(uri) instead of manually copying metadata and NoExtensions.

ProblemDetails now defaults its extension type parameter to NoExtensions. Both ProblemType and ProblemDetails gain const detail, instance, and extensions methods; reference-based conversion borrows type URI/title from the definition when possible. The crate enables nightly const-trait features (const_convert, const_destruct, const_trait_impl) to support const construction.

Tests and doctests are updated to use the new API and assert borrowing behavior with assert_matches!.

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