LLMGatewayKeySpec: add explicit Literal field to prevent silent typo bugs

[yrobla]

Context

Identified in PR #5142 review by @JAORMX and Copilot.

Problem

llmValueForSpec in pkg/client/llm_gateway.go has a default: case that writes the ValueField string verbatim as a literal value when it doesn't match any known resolver. This is currently used intentionally for gemini-api-key in the Gemini CLI config entry.

The foot-gun: a typo in any known ValueField (e.g. "GatwayURL" instead of "GatewayURL") silently writes the typo as a literal string into the user's settings file — no error, no test catches it.

Additionally, the doc comment on LLMGatewayKeySpec says "Exactly one of ValueField or Literal must be set", but the code does not enforce mutual exclusion (Literal wins if both are set).

Suggested fix

Add an explicit Literal field to LLMGatewayKeySpec:

type LLMGatewayKeySpec struct {
    JSONPointer    string
    ValueField     string // must be empty when Literal is set
    Literal        string // written verbatim; must be empty when ValueField is set
    ClearWhenEmpty bool
}

And validate at startup (or in llmValueForSpec) that exactly one of ValueField/Literal is non-empty. Unknown ValueField values should return an error rather than being silently treated as literals.

The Gemini CLI entry becomes:

{JSONPointer: "/security/auth/selectedType", Literal: "gemini-api-key"},

References

• PR Correct Gemini CLI LLM gateway config to proxy mode #5142 review comment by @JAORMX on pkg/client/llm_gateway.go
• PR Correct Gemini CLI LLM gateway config to proxy mode #5142 review comment by @JAORMX on pkg/client/config.go (line 163, doc contract nit)
• PR Correct Gemini CLI LLM gateway config to proxy mode #5142 review comment by Copilot on pkg/client/config.go (literal-value behavior undocumented)

[yrobla]

Already fixed in #5142 — the review feedback was addressed before merge. LLMGatewayKeySpec has an explicit Literal field, llmValueForSpec returns it verbatim and errors on unknown ValueField values, and all cases are covered by passing tests.