{agent, gateway, runtime}: restore versioned agent factories and budget controls

[XnLemon]

What changed

• Restore the versioned AgentFactoryRegistry and schema-v1 chain Agent kind while retaining the existing llm path.
• Persist and validate ordered Chain steps through Agent revision snapshots and PostgreSQL/MySQL codec and migration surfaces.
• Add tenant budget reservation, settlement, release, idempotency, and concurrency-safe PostgreSQL/InMemory ledgers.
• Connect model usage callbacks to Gateway budget settlement and audit cost/provider/model fields.
• Expose model pricing options and map budget failures to safe HTTP/audit errors.
• Cover budget arithmetic, lifecycle, in-memory contention, and PostgreSQL storage paths.

Context

This reintroduces the functionality from reverted PR #158 on top of current main, with direct-main alignment fixes and expanded budget coverage.

Compatibility

• Existing llm revisions remain compatible; unknown Agent kinds or schema versions fail explicitly.
• chain runs 2-32 tenant-authored steps sequentially and inherits the parent model, tools, generation settings, and runtime policy.
• PostgreSQL budget storage is selected by the normal PostgreSQL bootstrap; single-process/no-database composition uses the in-memory store.
• Root migration history adds feature-owned Agent-kind and budget-ledger changes; MySQL Agent schema support is included, while a MySQL budget ledger remains follow-up work.

Testing

• go test ./...

Fixes #156
Fixes #157

[codecov]

Codecov Report

❌ Patch coverage is 87.35005% with 116 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
trpcservice/runtime/budget/budget.go	85.79%	12 Missing and 12 partials ⚠️
trpcservice/gateway/dispatch_budget.go	79.54%	11 Missing and 7 partials ⚠️
trpcservice/runtime/budget/inmemory/inmemory.go	86.88%	8 Missing and 8 partials ⚠️
trpcservice/app/revision.go	81.33%	7 Missing and 7 partials ⚠️
trpcservice/gateway/dispatch.go	77.77%	9 Missing and 3 partials ⚠️
trpcservice/runtime/budget/postgres/store.go	94.82%	6 Missing and 6 partials ⚠️
trpcservice/gateway/dispatch_execution.go	69.23%	5 Missing and 3 partials ⚠️
trpcservice/bootstrap/bootstrap.go	71.42%	2 Missing and 2 partials ⚠️
trpcservice/agent/factory.go	97.22%	1 Missing and 1 partial ⚠️
trpcservice/agent/runner.go	81.81%	1 Missing and 1 partial ⚠️
... and 2 more

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