server: eagerly create prompts log directory at startup

[rankaiyx]

Overview

--log-prompts-dir is used to log prompts during debugging, but it requires the directory to exist at the time of the request.

If the directory is missing, every request triggers an std::ofstream open failure and an SRV_ERR log entry; this not only clutters the runtime logs but also renders the debugging feature unusable.

Create the prompts log directory after parameter parsing instead of relying on it existing at request time.

Uses std::filesystem::create_directories for correct recursive creation on all platforms.

The primary use case is for temporary file systems like /tmp/foo, to avoid having to manually create the directory every time the server reboots.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: YES . Collaborated with an agent based on Qwen3.6-27B to create the patch. Human reviewed and successfully tested.

[ngxson]

IMO this should better be inside arg.cpp

[rankaiyx]

IMO this should better be inside arg.cpp

Changed as suggested.
Also updated the docs.
Removed the CLI docs because that feature isn’t actually implemented.

After testing, it is working properly as expected.

[ngxson]

why did you remove the CLI?

[rankaiyx]

The CLI does not actually implement this functionality; the configuration item is only referenced on the server side.

[rankaiyx]

Moreover, the primary use case for this feature does indeed appear to be server-side.
If there is a need for a CLI in the future, it can be added back during implementation.

[rankaiyx]

If I want to adhere to the principle of minimal changes, I could also undo the second commit. what do you think?

[pwilkin]

This will be soon irrelevant anyway with #24948

[ngxson]

please undo any changes not mentioned in the description

[rankaiyx]

Okay, that's done.

[pwilkin]

Nooooo we let a PR with an EditorConfig checker error through, CISC is gonna kill us...

[pwilkin]

@rankaiyx could you maybe kill that single EditorConfig error as well?

[ServeurpersoCom]

diff --git a/common/speculative.cpp b/common/speculative.cpp
index cfe4130e5c..955ea43052 100644
--- a/common/speculative.cpp
+++ b/common/speculative.cpp
@@ -2311,7 +2311,7 @@ common_init_speculative_result::common_init_speculative_result(
     } else if (spec_mtp) {
         model_path = params.model.path;

-        LOG_TRC("%s: creating MTP draft context against the target model '%s'\n", <- It's a EOL whitespace here
+        LOG_TRC("%s: creating MTP draft context against the target model '%s'\n",
             __func__, model_path.c_str());

         llama_context * ctx_dft = llama_init_from_model(model_tgt, cparams);

CI fails

[rankaiyx]

CI Error Cause: The editorconfig check failed due to trailing whitespace at common/speculative.cpp:2304. This whitespace was already present in upstream/master (introduced by commit f5525f7), not by this PR. It was later fixed by commit c198af4 (#25410), but the CI ran against an older version of master that still contained the issue.

Fix: Rebased this branch onto the latest upstream/master, which already includes the whitespace fix. The CI should now pass.

Let's wait for the CI to turn green.