feat(go/plugins/anthropic): register latest Claude models

[adesinah]

This PR registers latest Claude models (opus 4.8/4.7/…) for Go↔JS parity.

Closes: #5494

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Code Review

This pull request centralizes the configuration of Claude model capabilities by introducing a shared modelOptions helper, mirroring the JS plugin's KNOWN_MODELS. It maps curated models to advanced capabilities (such as JSON output support) and integrates this helper into both ListActions and ResolveAction. Additionally, new tests and sample flows have been added to verify and demonstrate these capabilities. The review feedback points out an issue where versioned model IDs containing a date suffix (e.g., -YYYYMMDD) will fail the lookup in knownModels and fall back to default options, thereby losing their curated capabilities. A code suggestion is provided to strip this suffix before performing the lookup.

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[gemini-code-assist]

When a user requests a model using its fully-qualified versioned ID (e.g., claude-opus-4-5-20251101), modelOptions looks up the full versioned ID in knownModels. Since knownModels only contains the unversioned aliases (e.g., claude-opus-4-5), the lookup fails and falls back to defaultClaudeOpts. This means versioned model IDs will not receive their curated capabilities (such as JSON output support).

To fix this, we should strip the 8-digit date suffix (e.g., -YYYYMMDD) from the model name before looking it up in knownModels.

func modelOptions(name string) ai.ModelOptions {
	baseName := name
	if len(name) >= 9 && name[len(name)-9] == '-' {
		isDate := true
		for i := len(name) - 8; i < len(name); i++ {
			if name[i] < '0' || name[i] > '9' {
				isDate = false
				break
			}
		}
		if isDate {
			baseName = name[:len(name)-9]
		}
	}

	opts, ok := knownModels[baseName]
	if !ok {
		opts = defaultClaudeOpts
	}
	if opts.Label == "" {
		opts.Label = fmt.Sprintf("%s - %s", anthropicLabelPrefix, name)
	}
	return opts
}