quiche: Runtime-created paths do not inherit PMTUD configuration

[idy]

Background

When Config::discover_pmtu(true) is enabled, the initial path receives a
Pmtud instance because it is created with Some(config). Paths created after
the handshake do not: both the client probe_path() path and a server path
created for a newly observed 4-tuple call Path::new(..., None).

As a result, a connection that migrates to a new local or peer address cannot
run PMTU discovery or revalidate_pmtu() on that replacement path. This is
independent from #2565, which concerns selecting the wrong existing path's
PMTUD state while sending.

Goal

Make runtime-created client and server paths inherit the connection's effective
PMTUD enablement and probe-attempt configuration, while keeping PMTU probes off
an unvalidated path until QUIC path validation succeeds.

Non-goals

• Do not change the PMTUD search algorithm.
• Do not change path selection or active-path switching.
• Do not duplicate the send-path state correction proposed in PMTUD state is consumed by wrong path when sending on probing path #2566.

Code Changes Tree

quiche/
└── src/
    ├── lib.rs        # retain effective PMTUD settings and apply them to runtime-created paths
    ├── path.rs       # defer PMTU probes until a path has completed validation
    └── tests.rs      # cover PMTUD inheritance on client- and server-created runtime paths

Design

The connection retains the effective discover_pmtu and
pmtud_max_probes values, including values applied by the TLS configuration
callback. Each later client or server path receives a fresh path-scoped
Pmtud instance with those settings.

An unvalidated path still sends the normal bounded PATH_CHALLENGE/
PATH_RESPONSE traffic. PMTUD probing starts only after that path becomes
validated, so an oversized candidate probe cannot interfere with path
validation.

Test And Acceptance Criteria

Acceptance Criteria

• A client path created by probe_path() has PMTUD state when the connection
  enables PMTUD.
• The corresponding server runtime path also has independent PMTUD state.
• A runtime path does not send a PMTU probe before validation.
• Existing initial-path PMTUD behavior is unchanged.

Validation

• cargo +nightly fmt --all -- --check
• cargo test -p quiche runtime_paths_inherit_pmtud_configuration --lib