server, fix CI: read a small temp file in the read_file tool test [no release] (#32168)
* server: read a small temp file in the read_file tool test
test_tools_builtin_read_file read its own file, which grows with every
added test. On Windows the CRLF checkout puts it at 16664 bytes, over
the 16384 byte read limit, and the tool refuses it.
The test now writes a small file of its own and checks the exact
content read back.
* server: use a temp cwd in the x-tool-cwd tool test
test_tools_builtin_cwd_header read test_tools_builtin.py through the
x-tool-cwd header, which hits the same 16384 byte read limit on the
Windows CRLF checkout, and wrote its shell marker into the source tree.
The test now uses a temp directory as cwd, reads a small file from it
and checks the shell marker lands there.