refactor(mcp): remove writeEnabled param from Server.Start

[Ankitsinghsisodya]

Problem

Server.Start accepted a writeEnabled bool parameter and reassigned s.readonly from it:

func (s *Server) Start(ctx context.Context, writeEnabled bool) error {
    s.readonly = !writeEnabled
    return s.impl.Run(ctx, s.transport)
}

The instructions string is embedded into mcp.ServerOptions at New() time via instructions(s.readonly) — a static string baked in before Start is ever called. The s.readonly reassignment in Start had no effect on the instructions agents receive, but could silently make handler behaviour inconsistent with those instructions if the two callers ever passed disagreeing values.

cmd/mcp.go already passes WithReadonly(!writeEnabled) to New(), so s.readonly is set correctly before instructions() runs. The writeEnabled parameter on Start was therefore redundant and a latent footgun.

Fix

Remove writeEnabled bool from Server.Start and propagate the signature change through the call stack. WithReadonly in New() is now the single source of truth for readonly mode.

Files changed

• pkg/mcp/mcp.go — Start(ctx), removed s.readonly = !writeEnabled
• pkg/functions/client.go — MCPServer interface, StartMCPServer, noopMCPServer all drop the bool param
• pkg/mock/mcp_server.go — StartFn and Start drop the bool param
• cmd/mcp.go — StartMCPServer(cmd.Context()), no writeEnabled arg
• cmd/mcp_test.go — TestMCP_StartWriteable rewritten; removed unused context import
• pkg/mcp/mcp_test.go — server.Start(t.Context()), removed !readonly
• pkg/functions/client_test.go — StartMCPServer(t.Context()), removed true

Testing

All existing tests pass. Readonly correctness (instructions reflecting the correct mode) is covered by TestInstructions in pkg/mcp.

[knative-prow]

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[knative-prow]

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[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.83%. Comparing base (584c11e) to head (13ab958).

Files with missing lines	Patch %	Lines
pkg/mock/mcp_server.go	0.00%	3 Missing ⚠️
pkg/functions/client.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3765      +/-   ##
==========================================
- Coverage   57.03%   56.83%   -0.20%     
==========================================
  Files         182      182              
  Lines       21376    21375       -1     
==========================================
- Hits        12191    12149      -42     
- Misses       7953     8002      +49     
+ Partials     1232     1224       -8     

Flag	Coverage Δ
e2e	35.81% <0.00%> (+<0.01%)	⬆️
e2e go	?
e2e node	?
e2e python	?
e2e quarkus	?
e2e rust	?
e2e springboot	?
e2e typescript	?
e2e-config-ci	28.14% <0.00%> (-0.02%)	⬇️
integration	17.18% <0.00%> (+<0.01%)	⬆️
unit macos-14	45.09% <50.00%> (-0.01%)	⬇️
unit macos-latest	45.09% <50.00%> (-0.01%)	⬇️
unit ubuntu-24.04-arm	45.33% <50.00%> (-0.01%)	⬇️
unit ubuntu-latest	46.03% <50.00%> (-0.01%)	⬇️
unit windows-latest	45.13% <50.00%> (-0.01%)	⬇️

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[lkingland]

Thanks @Ankitsinghsisodya! Closing as a duplicate of #3758, which is already lgtm/approved and just needs a rebase. Same fix, same signature change.