Add stdin close to envd to correctly handle command that wait for stdin EOF

[ValentaTomas]

Note

Introduces a way to signal EOF to non-PTY processes and hardens stdin handling.

• Adds CloseStdin RPC to process.proto with generated clients/handlers in both internal and shared packages, and wires routing in processconnect files
• Implements Handler.CloseStdin() and guards stdin with stdinMu; WriteStdin now synchronized and returns "stdin not enabled or closed" when unavailable
• Exposes CloseStdin via service method in input.go that resolves a process and calls handler
• Regenerates protobufs: new CloseStdinRequest/Response messages and updated method/type indexes

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[levb]

LGTM