fix(webapp): eliminate SSE abort-signal memory leak #5479

Job	Run time
e2e / 🧪 CLI v3 tests (ubuntu-latest - pnpm)	1m 54s
typecheck / typecheck	6m 55s
sdk-compat / Bun Runtime	2m 10s
sdk-compat / Node.js 20.20 (ubuntu-latest)	2m 3s
e2e / 🧪 CLI v3 tests (windows-latest - npm)	3m 36s
sdk-compat / Cloudflare Workers	2m 4s
sdk-compat / Node.js 22.12 (ubuntu-latest)	1m 58s
e2e / 🧪 CLI v3 tests (windows-latest - pnpm)	3m 56s
sdk-compat / Deno Runtime	2m 9s
units / packages / 🧪 Unit Tests: Packages (1, 1)	7m 47s
e2e / 🧪 CLI v3 tests (ubuntu-latest - npm)	1m 43s
units / internal / 🧪 Unit Tests: Internal (2, 8)	7m 47s
units / internal / 🧪 Unit Tests: Internal (6, 8)	6m 40s
units / internal / 🧪 Unit Tests: Internal (4, 8)	5m 59s
units / internal / 🧪 Unit Tests: Internal (5, 8)	5m 46s
units / internal / 🧪 Unit Tests: Internal (1, 8)	6m 4s
units / webapp / 🧪 Unit Tests: Webapp (8, 8)	6m 5s
units / webapp / 🧪 Unit Tests: Webapp (4, 8)	6m 26s
units / webapp / 🧪 Unit Tests: Webapp (7, 8)	2m 42s
units / internal / 🧪 Unit Tests: Internal (3, 8)	7m 46s
units / internal / 🧪 Unit Tests: Internal (7, 8)	5m 38s
units / internal / 🧪 Unit Tests: Internal (8, 8)	2m 44s
units / webapp / 🧪 Unit Tests: Webapp (5, 8)	4m 28s
units / webapp / 🧪 Unit Tests: Webapp (2, 8)	4m 12s
units / webapp / 🧪 Unit Tests: Webapp (1, 8)	6m 16s
units / webapp / 🧪 Unit Tests: Webapp (3, 8)	3m 1s
units / webapp / 🧪 Unit Tests: Webapp (6, 8)	7m 48s
units / webapp / 📊 Merge Reports	1s
units / packages / 📊 Merge Reports	1s
units / internal / 📊 Merge Reports	1s
	2h 5m 40s

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