Don't serialize the GitHub chip's "Loading..." placeholder title

[luke-belton]

Problem

Pasting a GitHub PR/issue URL into the message editor inserts a chip labeled #<n> - Loading... while the real title is fetched. If the message is sent before the fetch resolves (or the fetch silently fails at the wrong moment), the placeholder is serialized into the message as title="Loading..." and rendered that way forever - e.g. <github_pr number="71827" title="Loading..." url="..." /> showing up in an agent conversation.

Changes

• Hoisted the placeholder into GITHUB_REF_PLACEHOLDER_TITLE in content.ts (single source of truth, used by buildGithubRefPlaceholderChip).
• contentToXml now serializes a still-unresolved chip with title="", the same shape as the existing failed-fetch fallback (#<n> labels), which the parser already round-trips as a title-less chip.
• The chip still shows "Loading..." in the editor while fetching - only the serialized message changes.

How did you test this?

• Added a regression test in content.test.ts that serializes a chip built by buildGithubRefPlaceholderChip and asserts the placeholder does not leak (this also pins the builder and serializer to the same constant).
• pnpm --filter @posthog/core test - 213 files / 2415 tests pass.
• pnpm --filter @posthog/core typecheck and biome lint packages/core - clean.
• I (the agent) did not run the desktop app; the change is confined to pure serialization logic.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit c9aaef9.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "Don't serialize the GitHub chip's "Loadi..." | Re-trigger Greptile}