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/extensions/manager.rs

@@ -3990,6 +3990,7 @@ mod tests {
         channels_dir: std::path::PathBuf,
     ) -> ExtensionManager {
         use crate::secrets::{InMemorySecretsStore, SecretsCrypto};
+        use crate::testing::credentials::TEST_CRYPTO_KEY;
         use crate::tools::ToolRegistry;
         use crate::tools::mcp::process::McpProcessManager;
         use crate::tools::mcp::session::McpSessionManager;
@@ -3997,8 +3998,7 @@ mod tests {
         std::fs::create_dir_all(&tools_dir).ok();
         std::fs::create_dir_all(&channels_dir).ok();
 
-        let master_key =
-            secrecy::SecretString::from("0123456789abcdef0123456789abcdef".to_string());
+        let master_key = secrecy::SecretString::from(TEST_CRYPTO_KEY.to_string());
         let crypto = Arc::new(SecretsCrypto::new(master_key).unwrap());
 
         ExtensionManager::new(