instrument: LLM provider adapters (M1.B + M8 port)

[mmercuri]

Summary

Ports the nine LLM provider adapters from the ateam reference
implementation onto the new layerlens.instrument base layer:

OpenAI, Anthropic, Azure OpenAI, AWS Bedrock, Google Vertex,
Ollama, LiteLLM, Cohere, Mistral

This rolls M1.B (the seven original providers) and M8
(Cohere + Mistral) into a single PR because they share the same
provider _base helpers (pricing, token counters, provider mixin)
and per-PR they would all be sub-1k LOC.

Scope

• src/layerlens/instrument/adapters/providers/_base/ — shared
  provider mixin (pricing.py, provider.py, tokens.py)
• src/layerlens/instrument/adapters/providers/{openai,anthropic,azure_openai,bedrock,google_vertex,ollama,litellm,cohere,mistral}_adapter.py
  — per-provider adapter
• tests/instrument/adapters/providers/ — unit + live tests for all
  nine providers (live tests skip when no API key is set)
• tests/instrument/adapters/test_pydantic_compat.py — shared compat
  surface used by every provider
• samples/instrument/{openai,anthropic,cohere,mistral}/ — runnable
  samples
• docs/adapters/providers-*.md — per-provider integration guide
• pyproject.toml — nine new optional extras
  (providers-openai, providers-anthropic, providers-azure-openai,
  providers-bedrock, providers-vertex, providers-ollama,
  providers-litellm, providers-cohere, providers-mistral) plus
  the providers-all umbrella

Blast radius

• Default pip install layerlens install set is unchanged. Each
  provider's heavy dep (openai, anthropic, boto3, etc.) is gated
  behind its own providers-<name> extra.
• No changes to existing public API surface.
• Importing layerlens.instrument still does NOT pull in any
  provider module (lazy registry lookup).

Test plan

• uv run pytest tests/instrument/adapters/providers/ -x —
  122 passed, 4 skipped (live-only without keys)
• uv run pytest tests/instrument/adapters/test_pydantic_compat.py -x
  — 62 passed
• Reviewer (m-peko) verifies provider scope rules + token
  accounting

Stacks on

• feat/instrument-base-foundation (M1.A) — required for the
  BaseAdapter surface this PR consumes.

Linear

LAY-3400 umbrella (M1.B + M8).

[mmercuri]

Linear: https://linear.app/layerlens/issue/LAY-3400 (LLM providers slice — 9 providers including Cohere + Mistral [M8]). Stacked on PR #93. Under Apollo M1 epic LAY-3423.