fix(runner): return error when -fpt filter fails to initialize

[singhvishalkr]

Summary

• When users specify -fpt\ (filter-page-type) flag, the dit classifier must be successfully initialized
• Previously, if the dit model was missing, httpx would only log a warning and silently fail to apply the filter
• This caused unexpected behavior where error pages would not be filtered as expected

Changes

• Return a descriptive error when the classifier cannot be initialized for -fpt\ usage
• Include guidance for users to run \dit data download\ to install the required model

Proof

Before (silent failure):
\
httpx -u jsdk.baidu.com -fpt error
https://jsdk.baidu.com # Error page NOT filtered
\\

After (clear error):
\
httpx -u jsdk.baidu.com -fpt error
FATAL: could not initialize page classifier for -fpt filter: dit: model.json not found (run 'dit data download' to install the model)
\\

Checklist

• PR created against the correct branch (main)
• Code follows project conventions
• Fix is minimal and focused

/claim #2495

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[Mzack9999]

Thanks for the attempt, but this doesn't actually fix #2495.

The reported bug is that -fpt error no longer filters error pages out of the box (it worked in v1.8.1 via -fep with zero setup). This PR only converts the silent dit.New() failure into a fatal error and points users at dit data download — a CLI that isn't documented anywhere in this repo. After this change, httpx -u ... -fpt error still doesn't filter anything on a clean install; it just refuses to run, which is a UX regression for existing users with working -fpt invocations in scripts/CI.

The real fix needs to live in the dit bootstrap path — embed the model in the package, ship it with httpx releases, or auto-fetch it into ~/.dit/ on first use (similar to how nuclei pulls templates). Closing this in favor of a proper fix for #2495.