feat: introduce MaxJsonSize trait for JSON encode buffer sizing

[MathiasKoch]

Summary

Update::max_size() (jobs and commands) and the provisioning register buffer were hardcoded values that overflow on non-trivial payloads — serde_json_core::to_slice returns PayloadError::BufferSize and the publish surfaces as JobError::Mqtt / Error::BufferSize. What's really a buffer-size bug looks like an MQTT/broker problem.

Observed on factbird-edge factory_reset: the post-cleanup IN_PROGRESS update carries a 3-participant report (~700 bytes JSON) and overflows the hardcoded 512-byte buffer every time.

Bumping the constants high penalises mqttrust/no_std users who pay for an embedded buffer they don't need. Make the size caller-known via the type system instead.

What's new

/// Upper bound on the JSON-serialized size of `Self`. Used by `ToPayload`
/// implementations on generic-Serialize wrapper types to size their encode
/// buffers.
pub trait MaxJsonSize: serde::Serialize {
    const MAX_JSON_SIZE: usize;
}

impl MaxJsonSize for () {
    const MAX_JSON_SIZE: usize = 4;
}

Serialize is a super-trait — a MAX_JSON_SIZE only makes sense for a JSON-encodable type, and use sites get a single tighter bound (S: MaxJsonSize) instead of the compound S: Serialize + MaxJsonSize.

Sites updated

Site	Before	After
jobs::Update<'a, S> (and report_progress / succeed_job / fail_job / publish_and_wait)	S: Serialize, hardcoded 512	S: MaxJsonSize, S::MAX_JSON_SIZE + framing
commands::Update<'a, R> (and succeed / publish_and_wait)	R: Serialize, hardcoded 2048	R: MaxJsonSize, R::MAX_JSON_SIZE + framing
provisioning::FleetProvisioner::provision* parameters	impl Serialize, hardcoded 1024	impl MaxJsonSize, P::MAX_JSON_SIZE + framing
transfer::status_details::StatusDetailsExt	(no size info)	gains const MAX_EXTRA_JSON_SIZE: usize
StatusDetails, CombinedStatusDetails<E>	(no MaxJsonSize)	impl MaxJsonSize; combined delegates to E::MAX_EXTRA_JSON_SIZE

Breaking change

Callers passing a custom status_details (jobs), result (commands), or provisioning parameters type need to add an impl:

impl MaxJsonSize for MyStatusDetails {
    const MAX_JSON_SIZE: usize = 1024;
}

StatusDetailsExt impls likewise need to declare MAX_EXTRA_JSON_SIZE.

Test plan

• cargo check (default features)
• cargo check --no-default-features --features std,mqtt_greengrass
• cargo test --no-default-features --features std,mqtt_mqttrust,commands_cbor,metric_cbor --lib — 88/88 still passing
• Hardware: factbird-edge factory_reset reaches a terminal state in cloud (CleanupCompleted progress reaches the cloud after the caller's status_details type declares an appropriate MAX_JSON_SIZE)

Follow-ups (not in this PR)

• JobError::Mqtt mapping in jobs/stream.rs discards the underlying error type. Surfacing the typed error would prevent future buffer-overflow / serialization issues from being misdiagnosed as broker problems.
• shadows::ShadowRoot::MAX_PAYLOAD_SIZE follows a similar per-trait const pattern. Could optionally be unified under MaxJsonSize later, but that's a wider refactor.