Phase 3d: add actionable configuration parse errors

[MGudgin]

This PR adds actionable configuration parse errors: the parser reports
malformed JSON separately from typed policy-shape errors and includes the full
policy path plus whole-file source location in every diagnostic, without leaking
secrets or letting untrusted text corrupt diagnostics or the state-aware stdout
response envelope.

Details

• New config_deserialize module: a path-aware deserialize layer that
  distinguishes JSON syntax errors from typed policy errors, attaching the full
  JSON path and whole-file line/column, escaping control and invisible
  formatting characters (incl. U+2028/U+2029 and bidi overrides), and redacting
  secret-bearing values.
• State-aware per-backend per-phase configs deserialize positionally from the
  retained request text, so typed errors carry whole-file coordinates matching
  base-config errors; navigation uses owned-key maps so escaped sibling keys
  never silently drop the location, with graceful fallback to the value-based
  path.
• Request loading borrows RawValue to discriminate one-shot vs state-aware
  without building a full untyped AST, masking the experimental subtree to
  preserve base-config source locations.
• Diagnostic routing keeps stdout reserved for the state-aware envelope and
  script output; parse and dispatch errors go to auxiliary sinks (log file /
  diagnostic pipe) only and are emitted exactly once. Filesystem-path and
  schema-version diagnostics escape untrusted text via the shared
  escape_diagnostic_text helper.
• Exec dispatch drops the parsed request (and its retained source text) before
  the blocking child run and stdio relay.
• Docs updated: docs/versioning.md, the state-aware API doc, and the
  copilot-instructions config-flow description.

Tests

• Added parser coverage: syntax-vs-typed classification, whole-file line/column
  (computed, not hardcoded), escaped-sibling-key location preservation, exact
  column, CRLF line stability, positional secret redaction, source-present
  locator fallback, malformed-telemetry single auxiliary emission with an empty
  primary buffer, and a pinned serde_json positioned-error display-suffix
  contract.
• cargo fmt --all -- --check: passed.
• cargo clippy -p wxc_common -p wxc --all-targets -- -D warnings: clean.
• cargo test -p wxc_common -p wxc: 514 + 26 passed.
• node scripts/versioning/check-schema-codegen.js,
  check-sdk-types-codegen.js, check-schema-versions.js: passed.

[Copilot]

Pull request overview

Adds a path-aware JSON deserialization layer to produce more actionable configuration parse errors (policy path + source location), while preserving the state-aware stdout envelope contract by routing parse/dispatch diagnostics only to auxiliary sinks.

Changes:

• Introduces config_deserialize to distinguish JSON syntax errors vs typed policy-shape errors, including full JSON paths, source locations, control-character escaping, and secret-value redaction.
• Refactors request loading/discrimination to borrow RawValue for state-aware discrimination (avoids building a full untyped JSON AST) and preserves byte/line locations by masking the experimental subtree.
• Adjusts logger + state-aware driver behavior to keep stdout reserved for envelopes/script output and avoid duplicating state-aware errors into primary stderr/buffer output.

Show a summary per file

File	Description
src/core/wxc/src/main.rs	Logs state-aware dispatch errors only to auxiliary diagnostic sinks before emitting error envelopes.
src/core/wxc_common/src/wire.rs	Improves root-shape error messaging (expecting) and documents optionality expectations for experimental.
src/core/wxc_common/src/state_aware_request.rs	Uses path-aware deserialize for per-backend phase configs and prefixes errors with experimental.<backend>.<phase>.
src/core/wxc_common/src/state_aware_dispatch.rs	Extends tests to assert envelope-ready error paths for typed state-aware backend configs.
src/core/wxc_common/src/logger.rs	Adds log_diagnostic_line to write only to auxiliary sinks (file/diagnostic pipe), and refactors log_line accordingly.
src/core/wxc_common/src/lib.rs	Wires in the new internal config_deserialize module.
src/core/wxc_common/src/config_parser.rs	Refactors request parsing/logging, adds RawValue discriminator + experimental masking for location preservation.
src/core/wxc_common/src/config_deserialize.rs	New: central path-aware deserialize + formatting/redaction/escaping for diagnostics.
src/core/wxc_common/Cargo.toml	Enables serde_json raw_value support and adds serde_path_to_error.
src/Cargo.toml	Adds workspace dependency on serde_path_to_error.
src/Cargo.lock	Locks serde_path_to_error.
docs/versioning.md	Documents the new “actionable parse errors” behavior and guarantees.
docs/state-aware-lifecycle/mxc-state-aware-sandbox-api.md	Documents auxiliary-only routing for state-aware parse-phase failures.
.github/copilot-instructions.md	Updates documented config flow to include config_deserialize.

Review details

• Files reviewed: 13/14 changed files
• Comments generated: 2
• Review effort level: Low