Fix concurrent map crash in dev server's Lambda bridge

[kieranm]

Fixes #6567

Problem

sst dev crashes with fatal error: concurrent map read and map write under concurrent Lambda invocations. nextChan and workers in cmd/sst/mosaic/aws/function.go are written by the event loop goroutine while the lambda runtime-API HTTP handlers read them from request goroutines, with no synchronization. A cold-start burst (e.g. the first page load of an app with many functions) reliably kills the whole dev process — most recently reproduced on 4.17.1 / macOS ARM64, matching the reports in #6567.

internal/runtime/maps.fatal({...})
github.com/sst/sst/v3/cmd/sst/mosaic/aws.function.func1(...)
        cmd/sst/mosaic/aws/function.go:90

Fix

One mutex guarding both maps, with two small locked accessors (loadNextChan, loadWorker) used by the handlers and the event loop. The handler-side loadNextChan also creates the channel on first access (same buffer size the event loop used), which removes the nil-channel wait for a worker whose init message hasn't arrived yet. The WorkerInfo.CurrentRequestID field — written by the event loop, read by the per-worker log pump goroutine — is guarded by the same mutex.

Channel operations stay outside the lock, so blocking behaviour is unchanged.

Reproduction / verification

The included regression test drives both sides concurrently with no infrastructure (a MessageInit for an unknown function id performs the racing map write and returns before any bridge/project access):

• Without the fix: go test -race -count=1 -run TestFunctionConcurrentInitAndNext ./cmd/sst/mosaic/aws/ fails in ~0.02s with the race pinned to the handler's map read.
• With the fix: passes.

Note the race detector isn't enabled in CI's go test, so the test passes harmlessly there on both versions; it exists so the race stays reproducible with -race.

Also verified against a real app (60+ functions) with a race-instrumented build of the CLI: before the fix, page-load bursts produced multiple data-race reports including the fatal site above; after, none.