fix: make agent cost accounting updates transactional

[sahu-virendra-1908]

Summary

This PR fixes a race condition in agent cost accounting caused by non-atomic multi-table database updates in server/src/core/heartbeat.ts.

Previously, updates to:

• agentRuntimeState
• costEvents
• agents.spentMonthlyCents

were executed as separate database operations outside a transaction. Under concurrent agent execution or process crashes, this could produce inconsistent financial state and allow budget enforcement to read stale spend values.

This change wraps all related accounting operations inside a single database transaction to ensure atomicity and consistency.

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Related Issue

Fixes #404

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Type of Change

• fix
• feat
• docs
• refactor
• perf
• test
• build
• breaking
• chore

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How Was This Tested?

• Local run
• Unit tests
• Integration tests
• Not tested (explain why)

Notes:

• Verified successful local server startup after changes.
• Verified transaction syntax and successful runtime execution.
• Confirmed no build/runtime errors introduced by transaction wrapper.

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CE & Security Check

• Targets GitMesh CE only (no EE code)
• No secrets or credentials committed

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Screenshots / Demos (if UI or UX)

N/A — backend concurrency and transactional consistency fix.

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Checklist

• Code follows project style
• Self-reviewed
• Tests updated/added where needed

[sahu-virendra-1908]

@Ronit-Raj9 @Parvm1102 @vibhor-5
Please review this PR.
This PR fixes a race condition in agent cost accounting caused by non-atomic multi-table updates in server/src/core/heartbeat.ts, which could lead to inconsistent financial state and potential budget enforcement bypass under concurrent runs.

If any improvements, corrections, or additional changes are needed, please let me know and I will update the PR accordingly.

[vibhor-5]

lgtm