Optional {placeholder} headers throw on missing arg (renderHeaders not optional-aware; breaks Bedrock invoke/converse)

[AtelyPham]

Summary

Capabilities that declare optional {placeholder} headers fail when the optional arg is omitted: renderHeaders runs interpolate over every header value, and interpolate throws missing required argument: <name> on an absent placeholder. Body fields are optional-aware (a {placeholder} body field is dropped when absent unless listed in the schema required), but header rendering is not — so optional headers are de-facto mandatory.

Concretely this breaks amazon-bedrock model.invoke / model.converse, whose schemas mark accept / contentType optional but whose request declares headers: { accept: '{accept}', 'content-type': '{contentType}' }. Calling them without those args throws before the request is sent.

General declarative-REST behavior (not AWS-specific); surfaced during #113 / #118.

Evidence

• src/connectors/adapters/declarative-rest.ts — renderHeaders maps each header through interpolate, which throws on a missing arg.
• src/connectors/adapters/amazon-bedrock.ts — model.invoke request headers { accept: '{accept}', 'content-type': '{contentType}' }; required: ['modelId', 'body'].

Repro

amazonBedrockConnector.executeMutation({ capabilityName: 'model.invoke',
  args: { modelId: 'anthropic.claude-3-sonnet-20240229-v1:0', body: {...} }, ... })
// → throws "missing required argument: accept"

Suggested fix

Make header interpolation optional-aware, mirroring renderObject for body fields: a header whose value is exactly {placeholder} is dropped when the arg is absent (unless the placeholder is in the capability's required list), instead of throwing.

Refs #113, #118.