Add TLS configuration options for VictoriaLogs connections

[mkobozev]

What does this PR do?

Adds two environment variables for connecting to VictoriaLogs instances that
serve a TLS certificate which is not trusted by the system certificate store:

Variable	Effect
VL_INSTANCE_TLS_CA_FILE	Path to a PEM-encoded CA bundle, appended to the system roots.
VL_INSTANCE_TLS_INSECURE_SKIP_VERIFY	Disables certificate verification entirely.

Why is this needed?

On-premise and internal deployments often terminate TLS with a certificate
issued by a private CA. Today the MCP server cannot talk to such an instance
over HTTPS at all.

VL_INSTANCE_TLS_CA_FILE is the intended fix for that case, and matches what
the VictoriaMetrics components already offer via -*.tlsCAFile.
VL_INSTANCE_TLS_INSECURE_SKIP_VERIFY is the escape hatch for development and
testing, mirroring -*.tlsInsecureSkipVerify.

Changes

• cmd/mcp-victorialogs/config/config.go: parses both variables and builds an
  *http.Client with the resulting TLS settings.
• cmd/mcp-victorialogs/tools/utils.go: GetTextBodyForRequest uses the client
  from the config. The *config.Config parameter was already in the signature
  but unused.
• cmd/mcp-victorialogs/main.go: logs a warning when verification is disabled.
• README.md: both variables added to the configuration table.
• cmd/mcp-victorialogs/config/config_test.go: subtests for defaults, valid and
  invalid values, a valid CA bundle, a missing CA file, a CA file with no
  certificates, and the mutually exclusive combination.

Notes

• Both options are disabled by default, so there is no behavior change for
  existing users. When neither is set, http.DefaultClient is used exactly as
  before.
• The custom transport lives on a client owned by Config rather than replacing
  http.DefaultTransport, so disabling verification cannot leak into unrelated
  outbound requests.
• The two options are mutually exclusive: InsecureSkipVerify would silently
  ignore RootCAs, so setting both fails at startup instead.
• The boolean is parsed with strconv.ParseBool and an invalid value fails at
  startup, consistent with how MCP_LOG_FORMAT, MCP_SERVER_MODE and
  MCP_HEARTBEAT_INTERVAL are handled.
• No new dependencies: everything used here is from the standard library.
• Happy to rename the branch or split the CA bundle support into a separate PR
  if you would prefer that.