Make Toolset.add chainable and flatten nested toolsets

[gulbaki]

Related Issues

• fixes Add multi server support to MCPTool and MCPToolset. haystack-core-integrations#1986

Proposed Changes:

• Modified add() to return self
• This allows chaining calls like toolset.add(a).add(b)
• Ensures nested Toolsets are flattened correctly

How did you test it?

• Added test: test_toolset_add_can_flatten_nested_toolsets

Release notes

Added in releasenotes/notes/toolset-add-chainable.yaml

[coveralls]

Pull Request Test Coverage Report for Build 15904837847

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 90.294%

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Files with Coverage Reduction	New Missed Lines	%
tools/toolset.py	1	92.98%

Totals
Change from base Build 15877046402:	0.02%
Covered Lines:	11712
Relevant Lines:	12971

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💛 - Coveralls

[sjrl]

Hey @vblagoje since you have the most context on Toolset I think it'd be best if you review this

[vblagoje]

@Amnah199 this one is not going into 2.15.1 - the bug is in mcp-haystack not core 🙏
Closing now, thanks for this contribution @gulbaki and apologies - I'll fix mcp-haystack where the bug originated