chains: export ChainCallOptions so custom Chain implementations can read options

[zxuhan]

Summary

ChainCallOption is currently defined as:

type ChainCallOption func(*chainCallOption)

where chainCallOption is unexported. That makes it impossible to implement the chains.Chain interface from outside the chains package while still respecting caller-supplied options — callers can pass a []ChainCallOption, but a custom implementation has no way to construct a *chainCallOption to invoke the functions against, and no way to read the resulting field values.

This PR exports the struct as ChainCallOptions (plural, to avoid colliding with the existing ChainCallOption function type), exports the *Set flags, and adds a public ApplyChainCallOptions helper:

type ChainCallOptions struct {
    Model    string
    ModelSet bool
    // ...
}

func ApplyChainCallOptions(options ...ChainCallOption) ChainCallOptions

Custom implementations can now write:

func (c *MyChain) Call(ctx context.Context, in map[string]any, opts ...chains.ChainCallOption) (map[string]any, error) {
    resolved := chains.ApplyChainCallOptions(opts...)
    if resolved.ModelSet { /* ... */ }
    // ...
}

Internal usage (NewLLMChain, GetLLMCallOptions) was switched over to ApplyChainCallOptions for consistency.

Compatibility

Renaming chainCallOption → ChainCallOptions and the xxxSet flags → XxxSet is non-breaking: external callers could not reference the private type, so no existing import path changes. The ChainCallOption function type itself is unchanged at the signature level.

Fixes #1085.

Test plan

• go build ./... clean
• go vet ./chains/... ./agents/... clean
• New TestApplyChainCallOptions_ExternalChain in chains_test (external package) demonstrates implementing chains.Chain from outside and reading caller options
• New TestApplyChainCallOptions_Empty covers the zero-value path
• Existing chains/ unit tests pass; only failing tests (TestLLMChainWithGoogleAI, TestConstitutionalChain) are pre-existing integration tests requiring external credentials