fix(messageConversion): deduplicate tool results for reused tool_call_ids (#12626) - #12627

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Description

When a provider reuses a tool_call_id across turns, Continue was emitting
duplicate tool_result entries for that ID on every subsequent request to the
provider. This corrupts provider conversation state, triggers schema validation
errors, and can cause runaway retry loops (the reproducer in #12626 shows 441
duplicate entries across 442 requests for a single reused ID).

Root cause: handleToolResult in messageConversion.ts matched tool results
against a global pendingToolCalls map without scoping to the current assistant
turn. When the same ID was reused in a later turn, the earlier turn's
toolCallStates entry still existed and got re-emitted on every serialization
pass by convertFromUnifiedHistory.

Two fixes in core/util/messageConversion.ts:

1. handleToolResult: guard the match so it only applies when the
   tool_call_id belongs to the current assistant turn's toolCallStates.
   After a successful match, delete the ID from pendingToolCalls so a reused
   ID in a later turn is treated as a fresh entry.

2. convertFromUnifiedHistory: track emitted tool_call_ids in a Set
   and skip duplicates on emit — a defensive second layer in case any other
   code path populates toolCallStates with a reused ID.

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

Screen recording or screenshot

N/A — this is a bug fix with no visual output. The reproducer in #12626 can be
used to verify: expected output changes from REPRODUCED to no duplicate
entries logged.

Tests

Added unit tests in core/util/messageConversion.test.ts covering:

• Tool results are emitted exactly once when tool_call_id is unique across turns
• Tool results are not duplicated when a tool_call_id is reused across turns
• convertToUnifiedHistory → convertFromUnifiedHistory round-trip produces
  a valid, non-duplicated message sequence for reused IDs

Summary by cubic

Deduplicates tool results when a provider reuses a tool_call_id across turns, preventing corrupted conversation state and retry loops. Fixes #12626.

• Bug Fixes
  • handleToolResult: only match IDs in the current assistant turn and remove the ID from pendingToolCalls after emit.
  • convertFromUnifiedHistory: track emitted tool_call_ids in a Set and skip duplicates as a defensive layer.

^{Written for commit 675749d. Summary will update on new commits.}