fix: tighten request handler result types on v1.x

[pragnyanramtha]

Summary

• add protocol method/result maps and route setRequestHandler return types through fixed-shape results for standard MCP requests
• include task-capable standard methods such as tools/call, sampling/createMessage, and elicitation/create while preserving custom request fallback typing
• add compile-time regressions proving invalid {} handler results fail, with positive coverage for task results and structured-content-only tool responses

Fixes #42.

Validation

• npm run typecheck
• npx vitest run test/server/index.test.ts test/client/index.test.ts test/server/elicitation.test.ts test/shared/protocol.test.ts -> 324 passed
• npx vitest run test/server/index.test.ts -> 67 passed
• npm run lint
• npm run build
• git diff --check

Review

• Independent review found no blockers after the final tools/call and elicitation/create tightening.
• tasks/result intentionally remains broad because its payload is method-dependent.

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Latest commit: a43ea63

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commit: a43ea63

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