feat(iroh): expose incoming connection limits on Endpoint::Builder

[random-ml]

Description

Adds endpoint-wide configuration for the number and buffered size of pending incoming connection attempts.

• Adds max_incoming, incoming_buffer_size, and incoming_buffer_size_total to Endpoint::Builder.
• Stores only explicit overrides, preserving the existing noq defaults when the methods are not called.
• Applies the overrides whenever the endpoint creates its default server configuration, including after Endpoint::set_alpns().
• Clarifies that the similarly named ServerConfigBuilder settings cannot affect an Incoming that already exists.
• Adds coverage for zero and nonzero pending-attempt limits, recovery after capacity is released, and application of all three overrides.

Closes #4545

API Changes

This adds three non-breaking fluent builder methods:

Endpoint::builder(presets::Minimal)
    .max_incoming(16)
    .incoming_buffer_size(64 * 1024)
    .incoming_buffer_size_total(256 * 1024);

The methods mirror the existing settings on ServerConfigBuilder, using the non-set_ naming convention already used by Endpoint::Builder. Existing behavior and defaults are unchanged.

Notes & open questions

The implementation stores optional overrides rather than duplicating noq's default values, so future default changes continue to flow through when applications do not configure these limits.

Testing

• cargo test --locked -p iroh --lib — 125 passed, 1 ignored.
• cargo test --locked -p iroh --all-features --lib — 131 passed, 1 ignored.
• cargo fmt --all --check -- --config unstable_features=true --config imports_granularity=Crate,group_imports=StdExternalCrate,reorder_imports=true,format_code_in_doc_comments=true.
• RUSTFLAGS=-Dwarnings cargo clippy --locked --workspace --all-features --all-targets --lib --bins --tests --benches --examples.
• RUSTFLAGS=-Dwarnings cargo clippy --locked --workspace --no-default-features --all-targets --lib --bins --tests --benches --examples.
• RUSTFLAGS=-Dwarnings cargo clippy --locked --workspace --all-targets --lib --bins --tests --benches --examples.
• RUSTDOCFLAGS=-Dwarnings cargo doc --locked --workspace --all-features --no-deps --document-private-items.
• git diff --check.

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All API changes documented.
• This PR was created by a human that thought critically about the
  proposed change and wrote an as clear and concise description as
  they could.
• This PR isn't slop, and is carefully crafted to do have the
  intented effect.