feat(agent-core): record context length before and after micro compaction

[7Sageer]

Related Issue

No related issue.

Problem

Full compaction already reports the estimated context length before and after compaction via the tokensBefore / tokensAfter fields on the compaction_finished telemetry event. Micro compaction only reported the token counts of the individual tool results it trimmed (beforeTokens / afterTokens), so there was no way to observe how the whole context length changed across a micro compaction, nor to compare the two compaction paths on the same axis.

What changed

Added two fields to the micro_compaction_applied telemetry event, mirroring tokensBefore / tokensAfter on compaction_finished:

• contextTokensBefore: estimated token count of the whole context (history) before trimming.
• contextTokensAfter: estimated token count of the whole context after trimming.

The existing beforeTokens / afterTokens fields are kept unchanged for backwards compatibility. Full and micro compaction can now be compared on the same axis. The existing micro compaction telemetry test now asserts the new fields.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

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