Merge pipeline config overlays by model ID

[hogeheer499-commits]

Description

Fixes #2004.

Config::overlay() currently appends decoder and vision pipeline entries even when an entry with the same model_id already exists. A partial component overlay can therefore create a duplicate entry with an empty filename, causing model loading to fail.

This change:

• reuses the matching pipeline entry and only appends unknown model IDs;
• preserves existing component filenames and session_options while applying a partial overlay;
• resets the active parser element before an append can reallocate the pipeline vector;
• applies the same semantics to decoder and vision pipelines.

The existing replacement behavior for run_options is unchanged.

Testing

• Added ConfigTests.PipelineOverlayMergesByModelId, covering:
  • two existing IDs, with only the second targeted;
  • preservation of the untouched entry, filename, log ID, and provider options;
  • addition of a new ID;
  • decoder and vision pipelines.
• Confirmed the regression test fails against current main (four entries instead of three) and passes with this change.
• Built the CPU RelWithDebInfo targets and ran the full CTest suite: 100% passed.
• Verified end to end with the Python binding: the existing ID now creates one profiled session, while a new ID still adds a second session.
• clang-format, lintrunner, and git diff --check pass.

AI assistance

OpenAI Codex (GPT-5) was used to assist with investigation, implementation, review, and testing. The resulting behavior was validated against current main with both unit and end-to-end tests.

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[Copilot]

Pull request overview

This PR fixes Config::overlay() pipeline overlay semantics so decoder/vision pipeline entries are merged by model_id instead of always appended, preventing duplicate entries with empty filename that can break model loading (Issue #2004).

Changes:

• Merge pipeline overlays by reusing an existing pipeline entry when model_id matches; only append for new IDs.
• Preserve existing pipeline component filename and session_options on partial overlays (while keeping run_options replacement behavior unchanged).
• Avoid potential invalidation by resetting the active per-entry JSON parser element before appending to the pipeline vector.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/config.cpp	Updates JSON overlay parsing to merge decoder/vision pipeline models by model_id and preserve existing per-model session_options on partial overlays.
test/config_tests.cpp	Adds a regression test asserting correct merge-by-model_id behavior for both decoder and vision pipelines.