geoprobe: create sockets on-demand instead of holding them open per target

[ben-dz]

Summary of Changes

• Replace persistent per-target sender pairs with on-demand socket creation: sockets are now created at probe time and closed immediately after, eliminating long-lived ephemeral port consumption
• Pinger tracks only target addresses (not sender objects), and creates/closes sender pairs in probeTarget() during each measurement cycle
• Extract createSenderPair() and probeTarget() helpers to consolidate duplicated probe logic from probeWorker and MeasureOne
• Introduce senderFactory function type for test injection, replacing direct senderEntry struct manipulation in tests

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	1	+102 / -155	-53
Tests	1	+76 / -63	+13

Net code reduction; the core change simplifies the pinger by removing persistent socket management.

Key files (click to expand)

• controlplane/telemetry/internal/geoprobe/pinger.go — replaced senders map[string]*senderEntry with targets map[string]ProbeAddress; added createSenderPair() and probeTarget() for on-demand socket lifecycle; simplified Close() to just clear the target map
• controlplane/telemetry/internal/geoprobe/pinger_test.go — switched tests from injecting senderEntry structs to using a mockFactory for senderFactory; updated all assertions from senders to targets

Testing Verification

• Unit tests updated to use senderFactory injection pattern; all existing test scenarios (dual-probe min RTT, one-fails-uses-other, both-fail, concurrent add/remove, MeasureOne) preserved
• Large-scale and concurrent test coverage unchanged