Add a per-call options channel on the request context

[OmarAlJarrah]

Problem

There's no per-call override for timeout, response-validation, or an ad-hoc credential. DispatchContext/RequestContext carry only instrumentation + payload.

Proposed change

Carry an immutable per-call options object on RequestContext, with applyDefaults merge semantics (per-field null-coalescing; nested timeout overlay). Keep the transport SPIs single-method. Model fields non-null with defaults (avoid nullable + !!).

Acceptance

• Per-call timeout/validation/credential override honoured
• Transport SPI unchanged
• Tests + apiDump

Dependencies

• Related Add a per-phase Timeout value type (connect/read/write/request) #41 — the per-call timeout overlay uses the per-phase Timeout type

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Priority: high · Effort: medium