refactor(core): compile the mock provider only with a testing feature

Phase 3a of docs/DESIGN.md. MockProvider moves to api/mock.rs, and it,
the embedded MSN fixture bodies, and the IDX_MOCK_* hooks compile only
with idx-core's new `testing` feature. idx-cli enables it as a
dev-dependency, so `cargo test` keeps the fixture-backed provider while
release binaries no longer embed fixture data or honor
IDX_USE_MOCK_PROVIDER. Parser helpers used only by the mock are gated
the same way.

The packaged-cli flake check no longer relies on the mock; it checks
an offline cache miss (JSON CACHEMISS, exit 3) instead.

Goldens unchanged; clippy is clean with and without `testing`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/idx-core/src/api/yahoo/mod.rs

@@ -11,6 +11,7 @@ use client::YahooClient;
 use map::{parse_fundamentals, parse_quote};
 use parse::parse_history_with_verbose;
 
+#[cfg(feature = "testing")]
 pub(crate) use parse::{
     parse_fundamentals_from_str, parse_history_from_str_with_verbose, parse_quote_from_str,
 };

crates/idx-core/src/api/yahoo/parse.rs

@@ -1,9 +1,16 @@
-use crate::api::types::{Fundamentals, Ohlc, Quote};
+use crate::api::types::Ohlc;
+#[cfg(any(test, feature = "testing"))]
+use crate::api::types::{Fundamentals, Quote};
 use crate::error::IdxError;
 
-use super::map::{parse_fundamentals, parse_history, parse_quote};
-use super::raw_types::{ChartResponse, QuoteSummaryResponse};
+use super::map::parse_history;
+#[cfg(any(test, feature = "testing"))]
+use super::map::{parse_fundamentals, parse_quote};
+use super::raw_types::ChartResponse;
+#[cfg(any(test, feature = "testing"))]
+use super::raw_types::QuoteSummaryResponse;
 
+#[cfg(any(test, feature = "testing"))]
 pub(crate) fn parse_quote_from_str(symbol: &str, raw: &str) -> Result<Quote, IdxError> {
     let chart: ChartResponse =
         serde_json::from_str(raw).map_err(|e| IdxError::ParseError(e.to_string()))?;
@@ -15,6 +22,7 @@ pub(crate) fn parse_history_from_str(symbol: &str, raw: &str) -> Result<Vec<Ohlc
     parse_history_from_str_with_verbose(symbol, raw, false)
 }
 
+#[cfg(any(test, feature = "testing"))]
 pub(crate) fn parse_history_from_str_with_verbose(
     symbol: &str,
     raw: &str,
@@ -25,6 +33,7 @@ pub(crate) fn parse_history_from_str_with_verbose(
     parse_history_with_verbose(symbol, &chart, verbose)
 }
 
+#[cfg(any(test, feature = "testing"))]
 pub(crate) fn parse_fundamentals_from_str(
     symbol: &str,
     raw: &str,