fix(gui): correlate id-less tool-call fragments by provider index

[canblmz1]

Description

applyToolCallDelta currently falls back to the most recently tracked tool call when a streamed continuation fragment does not contain an id.

The OpenAI SDK's streamed tool-call type requires an index while id is optional. Because Continue currently drops that index in fromChatCompletionChunk, interleaved parallel tool-call fragments can be applied to the wrong call.

This preserves tool_call.index through ToolCallDelta and ToolCallState and uses it to correlate id-less continuation fragments. Existing id-based correlation remains authoritative, and the legacy fallback remains unchanged for fragments that provide neither identity field.

Fixes #13223

Tests

Added an integration-style regression covering two interleaved parallel OpenAI tool calls and verified that each call retains only its own argument fragments.

Also re-ran the relevant existing GUI and core tool-call suites.

Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

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[canblmz1]

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