refactor: centralize test credential constants into testing::credentials (#829)

* refactor: centralize test credential constants into testing::credentials

Scattered test credential strings (API keys, OAuth tokens, crypto keys,
Telegram tokens, session tokens) across ~25 files made security auditing
harder and created unnecessary duplication. Centralize all test-only fake
credentials into a new `src/testing/credentials.rs` module with named
constants and a shared `test_secrets_store()` helper.

- Convert `src/testing.rs` to directory module (`src/testing/mod.rs`)
- Add `src/testing/credentials.rs` with ~30 named constants
- Replace hardcoded literals in 24 source files
- Deduplicate `test_store()` helper (was copy-pasted in 3 files)
- Leave leak_detector/shell/signature tests as-is (inline values
  aid readability for pattern detection tests)

[skip-regression-check]

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* refactor: replace real Telegram bot token with obviously fake test stub

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

* Update src/testing/credentials.rs

Co-authored-by: Copilot <[email protected]>

* Update src/testing/credentials.rs

Co-authored-by: Copilot <[email protected]>

* refactor: address PR review feedback on test credentials

- Fix TEST_CRYPTO_KEY doc comment ("32-byte hex" → "32-character key string")
- Rename confusing "real"/"fake" Anthropic constant names and values
- Change TEST_STRIPE_KEY from "sk-live" to "sk_test_fake123" to avoid scanners
- Use test_secrets_store() helper in orchestrator and http tool tests
- Clarify config_round_trip.rs doc comment about integration test visibility

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>
Co-authored-by: Copilot <[email protected]>

src/worker/api.rs

@@ -419,13 +419,14 @@ fn parse_finish_reason(s: &str) -> FinishReason {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::testing::credentials::TEST_BEARER_TOKEN;
 
     #[test]
     fn test_url_construction() {
         let client = WorkerHttpClient::new(
             "http://host.docker.internal:50051".to_string(),
             Uuid::nil(),
-            "test-token".to_string(),
+            TEST_BEARER_TOKEN.to_string(),
         );
 
         assert_eq!(
@@ -449,7 +450,7 @@ mod tests {
         let client = WorkerHttpClient::new(
             "http://host.docker.internal:50051".to_string(),
             Uuid::nil(),
-            "test-token".to_string(),
+            TEST_BEARER_TOKEN.to_string(),
         );
 
         assert_eq!(