fix(integrations): reserve our own slot in the client stack, and drop whole entries

Two review findings on this PR.

All three client-stack implementations appended our entry and then trimmed to
four, so whenever a caller already sent four entries the one dropped was exactly
the one the function exists to add. We vanished from our own stack while every
caller claim survived. The character cap was worse: slicing the joined string
severs an identifier, and the platform parses the fragment as a real client, so a
truncated tail arrives as a client literally named "me". Both caps now drop whole
entries and the reserved slot is ours, in the Python SDK, the TypeScript SDK and
pi-agent. mcp-server has the same fix on the platform branch.

The deepseek comment claimed the backend's allowlist recognizes DEEPSEEK_HARNESS.
This PR introduced that wording, replacing a neutral one. It is not true until
mem0ai/platform#3602 ships, so it now states the dependency.

Two pi-agent tests: our entry survives a full caller stack, and every surviving
entry is whole rather than a severed tail. Python side verified directly, a
4-entry caller stack keeps mem0-python and long entries are dropped whole.

312 passed 8 skipped, pi-agent 96, bundles clean, TS SDK builds.

Claude-Session: https://claude.ai/code/session_01C7tEmH86HAr7GoAAKCEHZb

integrations/deepseek-plugin/src/index.ts

@@ -26,8 +26,9 @@ export const name = "mem0";
 export const inject = ["tools", "systemPrompt"];
 
 // Tags writes so Mem0's backend attributes them to this integration in
-// telemetry. The backend's KNOWN_EVENT_SOURCES allowlist recognizes this value;
-// anything outside it buckets into "OTHERS".
+// telemetry. Values outside the backend's KNOWN_EVENT_SOURCES allowlist bucket
+// into "OTHERS"; this one is added by mem0ai/platform#3602 and reads as OTHERS
+// until that ships.
 const SOURCE = "DEEPSEEK_HARNESS";
 
 const DEFAULT_SEARCH_LIMIT = 10;