Add a display path to the shared executor for identifier-carrying requests #176

ADR-0006 (changed in #175 for #109) requires that a REST provider whose request URI, path or query, carries personal identifiers gives the shared executor a display path or route template (e.g. /api/search/nip/{nip}). The executor has no API for this yet; the ADR defers it to the first such provider.

Where the request URI leaks today

src/Http/Infrastructure/RestRequestExecutor.cs uses the absolute RequestUri (around :277) in:

• the url.path span tag;
• the warning and debug logs, including the error-response log;
• every status-error message (NotFound, BadRequest, 401/403, 408, …) and the timeout, connection and serialization error messages;
• ErrorResponseContext.RequestPath, passed to a provider's MapErrorAsync.

A 400's message also embeds the server's body, which may echo an identifier back.

NBP is unaffected: its paths and queries carry no identifiers.

Scope

• An additive executor API (e.g. a display path or template on the request) used for url.path, the logs, the error messages and ErrorResponseContext.RequestPath.
• A way for a provider to mask or drop a 400's server message.
• Tests that a URI containing an identifier appears nowhere in logs, tags, messages or metadata (ADR-0003).

When

With the first provider whose requests carry identifiers (likely MF or GUS), after the ADR-0010 gate. Not needed for NBP or for 1.0.