r/aws_bedrockagentcore_harness: add LiteLLM model and full skill union

[jesseturner21]

Description

Adds two harness model/skill features that exist in the Bedrock AgentCore control plane API but were missing from the Terraform schema.

• model.litellm_model_config — new model union variant (model_id, api_base, api_key_arn, max_tokens, temperature, top_p).
• skill — rebuilt from a path-only block into the full SDK union: path, s3, git (with auth), and aws_skills.

⚠️ Depends on #48655 — review that first

This PR is built on top of #48655 (the memory Unsupported Type fix). GitHub cross-fork PRs cannot target a fork branch as their base, so this PR is opened against main and its diff currently also contains the memory-fix commit from #48655. Once #48655 merges, that commit becomes part of main and this PR's diff will collapse to only the LiteLLM + skill changes.

Reviewers: the only commit unique to this PR is r/aws_bedrockagentcore_harness: add LiteLLM model and full skill union. The memory commit belongs to #48655.

Why the dependency is real: the service provisions a managed memory configuration by default, so a harness can't be created — and these acceptance tests can't pass — without #48655's flatten fix.

Split from #48556

Part of breaking up #48556 (size/XL) into focused PRs.

Honest caveats

• skill.path changes from Required to Optional (a widening; backward-compatible for existing configurations).
• Excluded additional_params / api_format on the harness model configs — already covered by open PRs Add additional params to Bedrock AgentCore harness #48498 / 🤖🤖🤖 [codex] Add Bedrock AgentCore harness api_format #48521.

Testing

TF_ACC=1 go test -v -timeout 60m ./internal/service/bedrockagentcore/ \
  -run "TestAccBedrockAgentCoreHarness_(model_litellm|skill_git)$"

--- PASS: TestAccBedrockAgentCoreHarness_skill_git (353.64s)
--- PASS: TestAccBedrockAgentCoreHarness_model_litellm (364.11s)
PASS
ok  github.com/hashicorp/terraform-provider-aws/internal/service/bedrockagentcore  364.271s

go build, go vet, gofmt clean.

Relations

Part of splitting #48556. Closes #48540. Depends on #48655.

[github-actions]

Community Guidelines

This comment is added to every new Pull Request to provide quick reference to how the Terraform AWS Provider is maintained. Please review the information below, and thank you for contributing to the community that keeps the provider thriving! 🚀

Voting for Prioritization

• Please vote on this Pull Request by adding a 👍 reaction to the original post to help the community and maintainers prioritize it.
• Please see our prioritization guide for additional information on how the maintainers handle prioritization.
• Please do not leave +1 or other comments that do not add relevant new information or questions; they generate extra noise for others following the Pull Request and do not help prioritize the request.

Pull Request Authors

• Review the contribution guide relating to the type of change you are making to ensure all of the necessary steps have been taken.
• Whether or not the branch has been rebased will not impact prioritization, but doing so is always a welcome surprise.