[adr] BiDi implementation boundaries (#17670)

* [docs] add design decision record process and template

* [adr] BiDi is an implementation mechanism, not a public API

* add examples with details as requested

* [adr] rework BiDi record: number by PR, drop index, reframe table and examples

* [docs] ADR 17670: BiDi implementation boundaries — supported API vs internal, composition access

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Co-authored-by: Diego Molina <diemol@users.noreply.github.com>

Author: titusfortner

docs/decisions/17670-bidi-implementation-boundaries.md

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+# 17670. BiDi implementation boundaries
+
+- Status: Proposed
+- Discussion: https://github.com/SeleniumHQ/selenium/pull/17670
+
+## Context
+
+Bindings use WebDriver BiDi to implement and extend the Selenium API. This record settles what
+users program against: where the supported API ends and the internal BiDi implementation begins.
+
+No binding conforms yet: each exposes BiDi implementation details from the driver object rather
+than keeping them internal.
+
+| Binding    | Current behavior |
+|------------|------------------|
+| Java       | Driver exposes the raw BiDi connection (`HasBiDi.getBiDi()`); protocol types are public |
+| Python     | `driver.network` / `driver.script` are the right high-level names, but return the low-level BiDi modules (`bidi`-namespaced) instead of neutral wrappers |
+| Ruby       | Driver exposes a BiDi accessor (`driver.bidi`) |
+| .NET       | Driver extension returns a BiDi type (`AsBiDiAsync()` → `IBiDi`) |
+| JavaScript | Driver exposes a BiDi accessor (`driver.getBidi()`) |
+
+All bindings also expose a BiDi-named option to enable the session (`enableBiDi` / `enable_bidi`),
+which can be renamed to be protocol-neutral.
+
+## Decision
+
+BiDi is an implementation mechanism, not a public API. Three decisions follow:
+
+1. **The supported API is protocol-neutral.** It never references BiDi — no BiDi types, and
+   nothing that hands a user a BiDi object. Users program against the Selenium API and its
+   high-level APIs, under the deprecation policy.
+2. **The BiDi implementation is internal and unsupported.** It faithfully implements the
+   commands, events, and modules of the BiDi spec (and could be generated from the CDDL). It
+   stays publicly reachable but is not governed by the deprecation policy: the spec is written to
+   be stable, but because it is not ours we do not guarantee it. Each binding marks it internal
+   by its own convention.
+3. **Low-level access is by composition, never off the driver.** The implementation is reached
+   by composing it with the driver — `BiDi::Protocol::Network.new(driver)` — never exposed as a
+   member of the driver, because anything reachable off the driver reads implicitly as supported.
+
+Everything else — orchestration, event dispatch, socket listening, transport — is an
+implementation detail, likely private, left to each binding.
+
+Marking a surface *Beta* does not satisfy decision 2 — Beta signals an API becoming supported,
+the opposite of internal.
+
+Illustratively (Ruby):
+
+```ruby
+driver.network.add_request_handler(...)                # supported — neutral, returns no BiDi type
+BiDi::Protocol::Network.new(driver).add_intercept(...) # internal — composed with the driver, unsupported
+driver.bidi.network.add_intercept(...)                 # not allowed — internal exposed as a driver member
+```
+
+## Considered options
+
+1. Expose the whole protocol as a public API (Rejected)
+   * the internal layer faithfully implements a living spec, so we cannot guarantee its stability — it can't be committed to as supported, public API
+2. A supported, separate "mid-level" API for working with the internal BiDi implementation (Rejected)
+   * protocol coupling — users build on BiDi-shaped concepts, the same trap CDP created
+   * two supported surfaces — a protocol-neutral high-level one and a BiDi-shaped mid-level one — leave users unsure which to reach for
+3. Expose the internal implementation as a driver member, e.g. `driver.bidi.network` (Rejected)
+   * a member of the driver reads implicitly as supported; composing the class with the driver instead keeps it distinct
+4. Internal implementation mechanism only, reached by composing with the driver (Accepted)
+
+## Consequences
+
+- Users never need to know which protocol services a command.
+- Existing public surfaces that fail conformance are brought in line per the deprecation policy.
+- Specific supported API behavior is specified in other ADRs.