fix(ai): omit maxTokens for Nova 2 at high reasoning effort - #51150

Issue for this PR

N/A. Found while testing default output limits for #51021.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Amazon Nova 2 Lite rejects any request that sets maxTokens while extended thinking runs at high effort:

maxTokens must be unset when reasoningConfig type is 'enabled' and maxReasoningEffort is 'high'

AWS documents the rule in Extended thinking in Amazon Nova 2: with "high", temperature, topP and maxTokens must be unset, because high-effort output can exceed 65k tokens.

Our high variant for Nova 2 sets additionalModelRequestFields.reasoningConfig.maxReasoningEffort: "high" through the request's HTTP body. The Converse protocol can see that body when it builds the request, so for Nova 2 model IDs (amazon.nova-2-*, with or without a region prefix) it now leaves maxTokens out in that case. Every other request keeps it.

• Only Nova 2: the rule is from AWS's Nova 2 docs. The older Nova 1.5 docs even show high effort with maxTokens: 40000. Our variants also send reasoningConfig to Grok 4.6 on Bedrock, and I checked live that it accepts maxTokens: 256000 at high and xhigh, so it keeps the limit.
• Only maxTokens: AWS also disallows temperature, topP and topK at high effort. An agent that sets them already fails today, and this PR doesn't change that.
• Why now: V2 only sends maxTokens on Bedrock when a hook sets it. feat(core): fit output limits to the context window #51021 sends a default output limit on every step, which would make every Nova 2 high request fail without this change.

How did you verify your code works?

• Unit test: builds models with the variant's body overlay, the way core does. Nova 2 high drops maxTokens; Nova 2 low and Grok 4.6 high keep it.

• Checks: the packages/ai Converse suite passes.

• Live against Bedrock, with the default output limits from feat(core): fit output limits to the context window #51021 applied:

  Variant	Requests	maxTokens sent	Result
  high, before	step and compaction	65,535 / 32,000	400, "maxTokens must be unset…"
  high, after	title, two steps, compaction	none	all 200
  low	title, two steps, compaction	65,535 / 32,000	all 200
  Grok 4.6 high / xhigh	step	256,000	200

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR