feat: add stdin support to providers, use stdin for OpenCodeProvider …

[letieu]

Pass query via stdin instead of a CLI positional arg in OpenCodeProvider to avoid shell argument size limits.

BaseProvider gains a _build_stdin hook (returns nil by default) that subclasses can override; make_request now forwards the result to vim.system's stdin option.

This is error:

Error in BufWriteCmd Autocommands for "<buffer=3>":                                                                                                                                                                              
Lua callback: vim/_system.lua:0: E2BIG: argument list too long                                                                                                                                                                   
stack traceback:                                                                                                                                                                                                                 
        [C]: in function 'error'                                                                                                                                                                                                 
        vim/_system.lua: in function ''                                                                                                                                                                                          
        vim/_system.lua: in function 'system'                                                                                                                                                                                    
        /home/tieu/.local/share/nvim/lazy/99/lua/99/providers.lua:76: in function 'make_request'                                                                                                                                 
        /home/tieu/.local/share/nvim/lazy/99/lua/99/prompt.lua:300: in function 'start_request'         

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Note

Medium Risk
Touches request execution for all providers by wiring optional stdin into vim.system, which could affect provider invocation if stdin/args expectations differ. Change is small and covered by updated unit tests for OpenCodeProvider.

Overview
Adds a new optional BaseProvider:_build_stdin() hook (default nil) and updates BaseProvider:make_request() to pass the returned value into vim.system as stdin.

Updates OpenCodeProvider to remove the query from its command args and instead provide it via _build_stdin, with accompanying test updates to assert the new behavior.

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