Add first-class UsageTap gateway provider (usagetapAdapter)

[PredictabilityAtScale]

Motivation

• Add native UsageTap LLM Gateway support as a distinct provider named usagetap modeled after the existing LLMAsAService adapter so prompt assets can target the first-party UsageTap gateway without changing llmasaservice behavior.
• Ensure prompt schema, defaults/override merging, raw passthrough, rendering, and provider registry treat usagetap as a first-class, provider-scoped option.
• Provide an OpenAI-compatible SDK helper so callers can construct an OpenAI client pointing at the gateway with runtime credentials that are never serialized into prompt artifacts.
• Keep existing UsageTap lifecycle/meters and LLMAsAService integrations unchanged so gateway calls are not double-metered.

Description

• Added a new provider adapter usagetapAdapter and SDK helper createUsageTapGatewayOpenAIConfig plus constants USAGETAP_GATEWAY_BASE_URL, USAGETAP_GATEWAY_DEFAULT_MODEL, and USAGETAP_GATEWAY_RESPONSE_HEADER_NAMES, and exported the new symbols from the package root and promptopskit/usagetap.
• Extended types and runtime options with UsageTapGatewayRuntimeOptions (runtime apiKey and optional idempotencyKey) and added UsageTapGatewayOpenAIConfig/UsageTapGatewayOpenAIConfigOptions types.
• Added schema support for provider_options.usagetap, optional rich customer attribution, and provider_options.usagetap.compress (and raw.usagetap passthrough), and wired these into defaults/override merging and raw merging logic.
• Reused the existing OpenAI chat mapping for messages/tools/streaming and implemented gateway-specific behavior: default model usagetap/standard, GPT-5 max_tokens → max_completion_tokens compatibility, ordered fallback_models → models mapping (avoid duplicating primary), injection of customer/feature/conversation/project/compress fields, raw-last override semantics, and runtime-only Idempotency-Key header emission.

Testing

• Ran the repository checks: npm run lint (tsc --noEmit) completed with no errors.
• Ran the unit test suite with npm test (vitest run) and all tests passed: 15 test files, 275 tests passed.
• Built distributables with npm run build (tsup) which completed successfully and produced ESM/CJS and DTS artifacts for the new usagetap entries.
• Ran package/type-quality checks npm run attw (node scripts/check-attw.mjs) and it reported "No problems found" for all entries including the new usagetap outputs.

Public API added: USAGETAP_GATEWAY_BASE_URL, USAGETAP_GATEWAY_DEFAULT_MODEL, USAGETAP_GATEWAY_RESPONSE_HEADER_NAMES, UsageTapGatewayRuntimeOptions, UsageTapGatewayOpenAIConfig, UsageTapGatewayOpenAIConfigOptions, createUsageTapGatewayOpenAIConfig, and usagetapAdapter.

Compatibility decisions: usagetap is a separate provider and does not replace or alter llmasaservice; UsageTap lifecycle/mediation helpers remain available but must not be used to wrap gateway calls (to avoid double-metering).

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Codex Task