retry: let the innermost retry interceptor set the attempt timeout

[boks1971]

A retry interceptor nested inside another one sets the attempt timeout again, and a timeout the caller sets still wins over both. This fixes livekit/livekit#4955.

Who sets the timeout

A request goes through the client's interceptors in the order they were added. The first one added runs first and wraps the rest:

caller → generic retry → egress retry → send

Each retry interceptor adds its attempt timeout to the request options. At the send, the options apply in order, so the last timeout in the list is used. In the protocol egress client, the generic retry comes from psrpc.timeout (300 ms in the issue), and the egress retry inside it uses 10 s, then 20 s and 40 s.

What #152 fixed

Each interceptor added its timeout at the end of the list. That overrode a timeout the caller set:

caller:          [5s]
generic retry:   [5s, 300ms]
egress retry:    [5s, 300ms, 10s]   → 10s, the caller's 5s is ignored

#152 put the interceptor's timeout at the front instead, so a caller's timeout comes later and wins.

Where #152 broke

An inner interceptor cannot tell the outer interceptor's timeout from a caller's. With no caller timeout:

caller:          []
generic retry:   [300ms]
egress retry:    [10s, 300ms]       → 300ms

The generic timeout now beat the egress one. Every StartEgress attempt timed out before a worker was picked, and the call failed after about 17 s.

The fix

Only the outermost retry interceptor sees the caller's options alone. It checks whether the caller set a timeout and passes the answer down in the context. If the caller set one, no interceptor adds its own. If not, each interceptor adds its timeout at the end, so the innermost one wins:

caller:          []
generic retry:   [300ms]            (notes: no caller timeout)
egress retry:    [300ms, 10s]       → 10s

caller:          [5s]
generic retry:   [5s]               (notes: caller timeout 5s)
egress retry:    [5s]               → 5s

	No caller timeout	Caller sets a timeout
Before #152	innermost wins ✓	caller ignored ✗
After #152	outermost wins ✗	caller wins ✓
With this fix	innermost wins ✓	caller wins ✓

TestNestedInnermostTimeoutWins fails without the fix.

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