fix(http): advertise Deno.serve keep-alive timeout

[lunadogbot]

Fixes #32497.
Closes bartlomieju/orchid-inbox#135

This adds a keepAliveTimeout option for TCP Deno.serve HTTP/1.1 servers. When set to a non-zero value, Deno configures Hyper with a timer-backed HTTP/1 header read timeout so idle keep-alive connections close on the configured schedule, and responses advertise the lifetime with Keep-Alive: timeout=N.

Verification:

• cargo fmt --check
• deno fmt --check tests/unit/serve_test.ts cli/tsc/dts/lib.deno.ns.d.ts
• cargo check -p deno_http
• cargo test -p deno_http test_handle_request
• cargo build -p deno
• ./target/debug/deno test --no-check -A --config tests/config/deno.json tests/unit/serve_test.ts --filter httpServerKeepAliveTimeoutAdvertisedAndEnforced
• cargo test -p deno --test integration serve:: -- --nocapture

Note: direct unit test execution required initializing tests/util/std so the repo import map could resolve local std test utilities.

[fibibot]

LGTM. The option is validated before reaching the op boundary, the Rust plumbing keeps it scoped to HTTP/1 serving, and the test covers both the advertised header and idle connection closure. CI is green.\n\n@bartlomieju this looks ready to merge.