feat: add sw_threading plugin for automatic cross-thread context propagation

[wu-sheng]

Summary

• Add new sw_threading plugin that patches Thread.__init__ and Thread.run to automatically propagate trace context across threads
• Update @runnable decorator to read snapshot from the Thread object at execution time instead of capturing at decoration time
• When sw_threading detects the target is @runnable-wrapped, it defers span creation to @runnable

This fixes the issue where @runnable only worked when applied inline during an active request. Module-level @runnable (the natural Python pattern) silently broke cross-thread trace linking because the snapshot was captured at import time when no trace context existed.

With the new plugin, users don't need @runnable at all — just pass a plain function to Thread and the context is propagated automatically:

def post():
    requests.post('http://provider:9091/users')

@app.route('/users')
def application():
    thread = Thread(target=post)
    thread.start()

Existing @runnable usage (both inline and module-level) continues to work.

Closes apache/skywalking#11605

Test plan

• New plugin test tests/plugin/web/sw_threading/ — plain function with Thread, validates CrossThread ref in expected data
• Existing Flask test (tests/plugin/web/sw_flask/) — uses inline @runnable, should still pass in CI
• CI full matrix validation

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