Harden AtlasCloud text tool-call parsing

[asim]

Summary:

• Harden text-encoded tool-call parsing to accept HTML-escaped tagged markup and spaced tool/function/name attributes.
• Add parser coverage for AtlasCloud/minimax delegate fallback formats.

Testing:

• go build ./...
• go test ./... (fails in this sandbox due loopback/network restrictions unrelated to this change: AtlasCloud stream test reached live endpoint without credentials; grpc loopback targets are blocked by the environment)
• golangci-lint run ./...

Closes #4244
Closes #4269

[Copilot]

Pull request overview

This PR hardens the agent’s text-based tool-call fallback parser so AtlasCloud/minimax outputs that use HTML-escaped tagged markup and/or spaced name= / function= attributes are still recognized and executed when structured tool_calls are absent.

Changes:

• Unescape HTML entities prior to parsing, enabling <tool_call ...>... formats to be detected.
• Broaden tagged tool-call parsing to accept whitespace around function = ..., and centralize tool-name extraction via a regex that tolerates spacing/quoting variants.
• Add focused unit tests covering spaced name = attributes and HTML-escaped <tool_call> markup.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
agent/text_tool_calls.go	Unescapes HTML before parsing; expands tagged tool-call/name parsing to handle spaced attributes robustly.
agent/text_tool_calls_test.go	Adds regression tests for spaced name = attributes and HTML-escaped tagged tool-call markup.

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