fix(core): clear consumed stdio read buffer

[sidsri14]

Summary

• Clear ReadBuffer's internal buffer when readMessage() consumes through the final byte.
• Preserve the existing zero-copy behavior for non-empty remainders.
• Add regression coverage that verifies the empty-buffer state is released and the next append keeps the incoming chunk instead of taking the concat path.

Fixes #2536.

Verification

• npx pnpm --filter @modelcontextprotocol/core-internal test -- test/shared/stdio.test.ts
• npx pnpm --filter @modelcontextprotocol/core-internal typecheck
• npx prettier --check packages/core-internal/src/shared/stdio.ts packages/core-internal/test/shared/stdio.test.ts
• npx eslint src/shared/stdio.ts from packages/core-internal

Note: pnpm --filter @modelcontextprotocol/core-internal lint currently fails on repo-wide Prettier checks for many pre-existing files in my Windows checkout, so I ran Prettier/eslint on the touched files directly.

[changeset-bot]

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Latest commit: 17f818d

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commit: 17f818d

[sidsri14]

Closing for now. I want to complete broader local validation and get explicit approval before keeping this PR open.