feat(identify): allow updating agent_version at runtime

[yexiyue]

Description

Config::agent_version is cloned into every Handler when a connection is established, so
mutating the configuration afterwards has no effect on already established connections. The only
way to change the advertised agent version today is to rebuild the Behaviour, which means
tearing down every connection.

This adds Behaviour::set_agent_version, which writes the new value to the config and notifies
every established connection through a new InEvent::AgentVersionChanged. The notification is
per connection (NotifyHandler::One) rather than NotifyHandler::Any, because a peer may be
reachable over several connections at once and each of them holds its own copy of the agent
version.

Setting the currently advertised value is a no-op. The method does not push by itself; combine it
with Behaviour::push to propagate the change immediately, otherwise peers pick it up with the
next periodic identify exchange.

The motivating use case is an application that encodes a user-visible device name into
agent_version. Renaming a device previously required restarting the whole node, which
disconnects every peer and interrupts in-flight transfers.

AI Assistance Disclosure

Tools used (required — write none if no AI was used): Claude Code

Attestation (required):

• I have read every line of this diff, understand what it does, and can explain it in review.

Notes & open questions

• The changelog entry is filed under the unreleased 0.48.0 heading, so no version bump is
  needed. I will add the See [PR ...] link once this PR has a number.
• set_agent_version deliberately does not push on its own, mirroring how
  Behaviour::push is already an explicit, separate step. Happy to change it to push
  automatically if you would rather have a single call.
• Verified against current master: cargo test -p libp2p-identify passes (4 unit + 9 smoke,
  including the new runtime_agent_version_update), cargo fmt --check clean, and
  cargo clippy -p libp2p-identify --all-targets reports no warnings for this crate.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates