fix(dsml): preserve schema strings and robust e2e reads

[SuperJJ007]

Summary

• preserve DSML parameters as strings when the tool schema says type: string, even if DeepSeek omits string="true"
• make the DSML E2E mock upstream read the full HTTP request body according to Content-Length
• add regression coverage for both cases

Tests

• python3 -m unittest test.test_dsml_shim.ParamTyping.test_schema_string_stays_string_without_string_attr -v
• python3 -m unittest test.test_proxy_dsml_e2e.HttpRequestReader.test_reads_body_split_after_headers -v
• python3 -m unittest test.test_dsml_shim -v
• python3 -m unittest test.test_proxy_units test.test_dsml_shim -v
• python3 -m unittest test.test_proxy_dsml_e2e -v
• bash test/run_all.sh