fix(custom tool): prevent Authorization prefix accumulation

[Epochex]

Fixes #35974.

Custom tool API-key auth was mutating runtime.credentials["api_key_value"] when adding the Bearer/Basic prefix. Because ToolRuntime can be reused across invocations (multi-step agent runs, retries, parallel calls), the prefix could accumulate over time (e.g. Bearer Bearer ).

This change:

• Computes the prefixed header value without mutating runtime.credentials.
• Avoids double-prefixing when the stored value is already prefixed.

Tests:

• uv run --project api pytest -q api/tests/unit_tests/core/tools/test_custom_tool.py -k assembling_request

[Epochex]

Actions checks haven’t started for this fork PR yet (workflow runs show “Action required”).

I ran the relevant unit test + ruff locally and they pass:

• uv run --project api pytest -q api/tests/unit_tests/core/tools/test_custom_tool.py -k assembling_request
• uv run --project api ruff check api/core/tools/custom_tool/tool.py api/tests/unit_tests/core/tools/test_custom_tool.py

Could a maintainer please approve/run the workflows so CI can execute?