fix: calculate contextTokens as cumulative sum of all conversation tokens

[roomote]

Summary

This PR fixes the incorrect token counting in the context window display that was causing users to see unrealistic values like 0 or under 100 tokens.

Problem

The contextTokens calculation in getApiMetrics was only looking at the last API request's tokens instead of calculating the actual cumulative context window usage. This led to:

• Incorrect token count display (showing 0 or very low numbers)
• Context window exceeded alerts despite low displayed token count
• Chat history condenser not working properly

Solution

Changed the contextTokens calculation to:

1. Sum all tokens from all API requests in the conversation
2. Properly handle condense_context messages by resetting to the new context size
3. Continue accumulating tokens after condensation

Changes

• Modified src/shared/getApiMetrics.ts to calculate cumulative context tokens
• Updated tests in src/shared/__tests__/getApiMetrics.spec.ts to verify correct behavior

Testing

• All existing tests pass
• Added new tests to verify cumulative token calculation
• Added tests for handling condense_context messages

Fixes #6004

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Important

Fixes contextTokens calculation in getApiMetrics to sum all conversation tokens and handle condense_context messages correctly.

• Behavior:
  • Fixes contextTokens calculation in getApiMetrics to sum tokens from all API requests.
  • Resets contextTokens after condense_context messages and continues accumulation.
• Code Changes:
  • Modified getApiMetrics.ts to track cumulative context tokens and handle condense_context messages.
  • Updated tests in getApiMetrics.spec.ts to verify cumulative token calculation and condense_context handling.
• Testing:
  • Added tests for cumulative token calculation and condense_context message handling.
  • Ensured all existing tests pass.

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[daniel-lxs]

Should be solved by #6019