config: prefix the remaining setters with with_ and hide Limits' fields

[iainmcgin]

Fixes #247

Renames the four config setters that carried no prefix — Limits::{max_request_body_size, max_message_size, element_memory_limit} and CompressionPolicy::min_size — to with_-prefixed forms, and makes the three Limits fields private with plain accessors.

Breaking, so it wants the 0.9.0 window, and taking both halves at once means the type is only broken once.

The naming was the smaller half. Every other config type in the crate uses with_, so a user who has learned it on ClientConfig or Server reaches for limits.with_max_message_size(..), gets method-not-found, and has to read the source. The overlap with the public fields was the sharper problem: field and method shared an identifier, so there were two ways to set each value, while #[non_exhaustive] (which Limits also gains in 0.9.0) rules out the struct-update syntax that public fields would otherwise buy. ClientConfig — private fields, with_ setters, plain accessors — is the shape this now follows, including the doc convention of cross-linking each setter and accessor to its partner.

The rename is not a silent break in either direction. A call site that kept the old setter name now resolves to a no-argument accessor, so passing a value is an arity error; a bare field read hits a private field. Every migration path is a compile error, which is what you want.

Issue #247 as I filed it claimed there were exactly three unprefixed setters. There were four — CompressionPolicy::min_size is the same shape and is called on the client side, so the issue's framing of these as "server config setters" was also wrong. Both corrected; the changelog fragment now claims only that these were the last config types whose setters were unprefixed, which is true. ClientConfig::json, proto and compress_requests remain unprefixed and are deliberately out of scope here.

CompressionPolicy needed the rename only — its fields were already private, and it has no read path to collide with the new name, so no accessor was added.

Ten Limits call sites and seven min_size sites are updated across the library, tests, conformance harness, rustdoc examples and the guide, along with the ElementMemoryLimitExceeded error text, which names the setter an operator should raise. One pre-existing unreleased changelog fragment is edited because its Rust example called a renamed setter, and one stale guide reference to a method that never existed is corrected.

One note on CI: two handler::tests element-budget tests fail on main right now, independently of this change — fixture rot from buffa 0.9.1, fixed by #239.