openai: add CustomHeaders to ChatModelConfig for parity with agenticopenai

[MOWmsc]

ChatModelConfig now exposes CustomHeaders to set extra HTTP headers on every request. The underlying openai.Config and genRequest() already support CustomHeaders mapping, so no changes are needed in the acl library.

Changes

• chatmodel.go: Add CustomHeaders map[string]string field to ChatModelConfig, and map it to openai.Config.CustomHeaders in NewChatModel
• option.go: Add WithCustomHeaders option function for per-request header overrides (consistent with agenticopenai.ChatConfig naming)
• chatmodel_test.go: Add unit tests verifying config mapping

How it works

The full CustomHeaders pipeline already exists in the lower layers — this PR just exposes the configuration entry point:

ChatModelConfig.CustomHeaders (NEW)
    ↓
openai.Config.CustomHeaders (already exists)
    ↓
genRequest().specOptions.ExtraHeader (already works)
    ↓
openai.WithExtraHeader() → reqOpts (already works)
    ↓
HTTP Request Headers (already works)

Fixes #1048

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