feat(plugin): add tool result content API

[kitlangton]

Summary

• add branded Tool.result({ output, content }) plus context.progress({ output, content }) to V2 plugin tools
• wire tool progress through the registry into replayable session tool progress events
• migrate built-in tools to explicit output/content results and keep bulky text out of structured output where possible

Stack

• stacked on refactor(plugin): move tool implementation to plugin #34665 (tool-package-boundary)

Testing

• bun typecheck (packages/plugin)
• bun typecheck (packages/core)
• bun typecheck (packages/server)
• bun typecheck (packages/sdk-next)
• bun run test -- test/session-runner-tool-registry.test.ts test/session-runner.test.ts test/session-tool-progress.test.ts test/tool-shell.test.ts test/tool-subagent.test.ts test/tool-skill.test.ts test/tool-webfetch.test.ts test/tool-websearch.test.ts (packages/core)
• bunx prettier --write touched files
• bunx oxlint touched files (warnings only; no errors)
• pre-push bun turbo typecheck

Local note

• packages/sdk-next: bun run test -- embedded.test.ts still fails one local live-notification test due configured provider auth/timeout; the embedded tool fixture test passes