fix: track origins by client map key in Agent teardown

[emmayusufu]

This relates to...

Fixes #5529

Rationale

Agent#closeClientIfUnused reads client[kUrl].origin for every dispatcher in the client map, but the factory option is public API and nothing guarantees a factory-built dispatcher sets kUrl. ProxyAgent's own factory registers Http1ProxyWrapper (and Socks5ProxyAgent), neither of which does, so since 8.7.0 a plain-http request through a proxy crashes with Cannot read properties of undefined (reading 'origin') on the disconnect / connectionError teardown path.

Changes

The teardown closure already holds the origin string that was added to kOrigins and the map key derived from it, so the bookkeeping doesn't need kUrl at all: check the remaining map keys for the origin (including the #http1-only variant) and delete the captured origin string. This is the third option suggested in the issue.

A side effect worth noting: kOrigins.delete is now symmetric with the add. Previously add used String(opts.origin) while delete used kUrl.origin, which diverge when opts.origin is a URL instance (trailing slash), so those origins were never removed and counted against maxOrigins forever.

The stats getter still reads dispatcher[kUrl].origin and has the same fragility with kUrl-less dispatchers; left alone here since changing its keys is a visible behavior change. Can follow up if there's interest.

Features

N/A

Bug Fixes

• ProxyAgent no longer crashes the inner Agent on connection teardown for plain-http (non-tunneled) targets.

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Documentation is not needed
• Lint passed