docs: plan injectable request reply callbacks

[bougyman]

Decision requested

Approve the proposed request/reply callback injection design before implementation begins.

Proposed design

• Resolve request/reply callbacks from service configuration through a callable resolver.
• Preserve the current NatsRequestReplyCallbacks behavior as the default resolver result.
• Let applications supply an ordinary callback object that responds to #callbacks; it must not inherit from or mix in Leopard code.
• Let application-composed policy decide logging and response behavior from the complete failure, including any status or other classification.

Non-goals

• No runtime code, tests, or README behavior changes are in this PR.
• This does not prescribe an application's failure type, status accessor, or status-policy mapping.

Acceptance criteria for follow-up implementation

• Existing services retain current request/reply responses and error logging by default.
• A service configures a resolver without monkey-patching or overriding Leopard internals.
• The resolver receives logger: and returns a callback object with #callbacks.
• A policy may choose a log level or no log entry for any failure status without changing respond_with_error(failure).
• Exceptions retain their current behavior unless a supplied callback policy deliberately changes them.

[gabeodess]

That should suite my needs!

[gabeodess]

Would the update use @failure_log_policy as the default so we could inject that without overwriting callbacks?

[bougyman]

afaa5b2 Like sew?